I am running a campaign which has a fair bit of mystery/intrigue and is based in a port city. Level 1-3 were a starter campaign (from the Wildemount source). The players are level 4 now and have reached the port city, established their base, had a few encounters and reached level 4. Level 3-5 are planned to be about learning the city, the factions, Problems With the City and also the Major Threats to the city.

By 5+ they will have chosen sides and also which Problem or Threat they wish to focus on, and at that point the campaign will be more focused and less sandboxy - but they will be several levels away from solving whatever the issue is. Several levels away from defeating a boss or being rewarded for service by Someone Important, etc.

Which brings me to my question: I want to reward them with some kind of magical items now, while keeping in mind that they'll be getting more in the future and I don't want to break the game by giving them too much. The DM guide discusses random magical item tables and such, but (and I may be wrong) not a lot about what/how much is appropriate, or how often?

There will be an opportunity in the next session or maybe the one after (depending on how distracted they get, and boy do they get distracted easily) to give them some fun stuff, and I'm curious how I can decide what to give? Should I pick a weak random table and roll dice? Should I go through lists and pick things each would really like, using judgement not to break stuff? Is level 4 too early for +1 gear? Should it all be consumable stuff or...

Any advice on this appreciated, as I am new to dnd/dming, as are more than half of the players. I already screwed up with loot at one point and they ended up with way too much gold at L3 (which I dealt with by creating a gold sink in the form of buying a building/base to live in once they reached the city). But I'd like to avoid a similar mistake with items that will be more difficult to take away later on.

 Xanathar's Guide to Everything has a useful section of awarding magic items.

I don't think it is too early to provide generic +1 weapons or some other +1 armor, but I wouldn't shower the whole party with it.  I have found that a potion or a scroll, being a one-use item, is a low risk for the DM because if it breaks a single encounter, well that not a big deal, but being a single use item it won't break a whole story or campaign.

A Bag of Holding is very common, is quite useful, but won't break a campaign.  I have discovered that Elven Chain is an awesome item for a player limited to light armor because they may use it as if proficient although they are not proficient with medium armor.  Frequently Elven Chain starts with +1, but it also allows the player to break the AC=12 cap of light armor.

A little more adventurous is a +1 weapon that also has a property that may be used 1, 2 or 3 times per long rest.  Such weapons might deal 1d6 lightning damage once per long rest, or once per day being recharged at dawn.

Silvered weapons are also a step up from other mundane weapons.

There are also a number of low level magic items such as the Pipes of Haunting, which when played by a character with proficiency deliver a fear spell for one minute.

Another great reward from the old days was an old spell book.  This allowed your wizards to increase the spells in their own book without having to track down a higher level wizard (and possibly pay for the access).

I recommend you simply read the DM guide with a pad of paper and write down the names of everything you would consider letting them have.  Just doing that is fun when you have some time.  It can also give you some good inspiration for an encounter or a whole story arc.

If you feel uncomfortable just give them those magic items with charges. It is a common and easy way to give the PC's items that will eventually run out of juice. This will allow you, as new DM, to get a glimpse of how your players will use such items. And the impact of the item on your game. Which then allows you to assess and decide in the future when to give items. Even with Xanathar's Guide...it is still a guideline. 5e is made without the use of magic items in mind. So when you start giving it out to your players. The DM needs to make adjustments to encounters anyway. 

I agree with the potions and scrolls comment. My now level 4 party has like 7 potions and 4 scrolls. Some of the potions have yet to be formally ID'ed (I do not allow a single sip to ID them fully, but I give hints).

The other thing I would say is... I usually give out nice magic items as loot (reward) for difficult things done on-game, like fighting a boss monster or overcoming a tough puzzle.

I like to see more mundane magic items handed out... such as the common magic items you can find in Xanathar's Guide. A lot of them get forgotten or never get fully utilized, but they have the potential for a lot of fun little moments.

Quote from TransmorpherDDS >>

I like to see more mundane magic items handed out... such as the common magic items you can find in Xanathar's Guide. A lot of them get forgotten or never get fully utilized, but they have the potential for a lot of fun little moments.

 I recently used DDB to look up all common magic items, and then all uncommon magic items, and then chose a few which would be of some use to the party.

Thanks for all the replies. The Xanathar's link is definitely helpful. I just bought Xanathar's last week too, so I feel a bit silly for not noticing that chapter.

I'll start by awarding the poor barbarian with a +1 weapon of some sort (she misses a lot).

The wizard can have something common-but-fun from Xanathar's list because I recently said yes to "Can my character buy and learn the dragons breath spell", which in hindsight was a reward all on its own. His owl familiar was strafing a barge full of smugglers with what seemed like napalm. Quite a fun evening.

The paladin is min-maxed so I'll award something fun/useful rather than powerful.

The arcane trickster rogue that doesn't like actual physical confrontation/combat... heh. I think I'll just ask him what he wants because I'm sure it'll be something fun rather than game-breaking.

And the bard... not sure. Xanathar suggests two major items by level 4. The bard felt kind of useless and unhappy with the mechanics of his character up until level 4 (but liked the rp so he stayed with it). Last week when I bought Xanathar's, I suggested he retcon his bardic college choice and pick one of the new ones from Xanathars. And Sunday night he seemed pretty happy with the class, but he might be a good candidate for an item with more impact.

Go poke around in the homebrew

There is already a lot of good answers in here! 

I usually look at where the party is having trouble with and try to help them in that way. Is the tank getting hit to hard every fight? Maybe some magical armor/shield.

Are they trying to haul all their loot but no sufficient way? Bag of Holding or other opportunities. 

Some items are fun but also exploitable... try and see how they go on with some items, if they exploit and it breaks the game there are always ways to fix that ingame.

I used to be really cautious about giving my players magical items, but after talking with some of my players I do more what MusicScout said because my players ended up really weak over time.