Hi, I'm new to DM'ing and am currently running a campaign with 4 players, 3 of whom are spellcasters without much by way of hit points or AC, but can deal a lot of spell damage quite quickly. 

I have been struggling to make encounters that I feel challenge the party. If I give them enemies that are stronger and do a lot of damage, they risk KO'ing a player with a single crit, but anything weaker and they are quite quickly dispatched with a couple of higher level spells.  Having multiple enemies helps balance this out a bit, but I don't necessarily want ever encounter to just be a swarm of enemies. 

Any suggestions to help with this? Like I said, I'm a new DM so sorry if this is a stupid question. 

Quote from LuckyDogg >>

 ...If I give them enemies that are stronger and do a lot of damage, they risk KO'ing a player with a single crit...

 This isn't a bad thing.  You want to make things challenging and your players probably want it as well.  Challenge involves risk, which includes death.  Luckily death isn't permanent in D&D.  Throw a bad guy with advantage vs. saves at them, or something with a Globe of Invulnerability cast on them.  Bad guys have brains and will try and negate the good guy's abilities from time to time.  This will give your 1 non caster a time to shine.  Just make sure to include the occasional closely packed horde of pions for the fireball lobbing wizard.  

Also don't be afraid to tweak things.  Nothics aren't that scary, until you make their rotting gaze do psychic damage and cause someone to lose one spell slot for the day as their magical energies are stolen by the Evil Eye'd freak.

tl;dr  Try and kill them every now and then it's fun.

Counterspell, Antimagic Field, Wall of Force, etc... lots of things in the toolkit to properly deal with a caster-heavy group.

Focus your enemy composition less on damage dealing, and more on control and mitigation. Shut down their ability to nuke things so easily. Throw in the occasional enemy with a Spellguard Shield, but make them as a cursed variant that crumbles to dust when attunement ends (the NPC dies). Use environmental effects that put the party on the back foot. Assault their senses; they can't target something they can't see. Use paralyzing poisons.

Most important of all: use cover.

Kill them all and let them roll new characters.

I kid. Have you tried the Encounter Builder? It can help gauge an encounter's overall difficulty versus player count and level. Aside from that, I always advocate letting the dice fall where they may based on the players' decisions. Almost all spellcasters get access to Mage Armor and/or Shield, and some can even wear armor. If they built a bunch of glass blasters with no thought of how to keep themselves alive, that's on them. Having them go down often also teaches players to evaluate new combat tactics and use spells beyond hurling a firebolt. In the past I have let a TPK happen only to have the characters remain alive due to enemy choices or divine intervention. As the DM you always have that as an option, but keeping the PCs alive is their responsibility, not yours.

how many encounters are running per adventuring day? If your only running 1 thats why you are having so many issues balancing them.  Try running 2-3 lesser encounters designed to use up their resources (spell slots) and then 1 lethal encounter.

Remember that the stats are just a guide. I would consider keeping the HP on the harder monsters, but dialing down the damage the monsters can do. Or dialing the HP up on the easier monsters that do less damage. It's better if you can have a story reason for the change, like a set of highly trained goblins in the service of the local BBEG, or else some bigger foes who have been diminished by age, or a curse, or illness. 

I'd also do like jdahveed suggested and teach them to try other tactics, and make them realise they need to prep some protective spells or get hold of some armour.

Mmm... shadow CR 1/2 so how many would that be versus your group? 

edit - I just had a little peek at the encounter builder for the first time and its suggesting for four 4th level characters a Medium difficulty encounter of 5 shadows worth adjusted 1000xp from a daily encounter budget of 6800 xp
I'm pretty sure that would be quite challenging and memorable encounter

For single "Boss" type enemies, you can give them Legendary Resistance and Legendary Actions. And demons/devils often have innate spell resistance.

Just make bad guys stuff that they would run into. Making your bad guys just to fit the group they are fighting against just kinda screams meta-gaming and trying to beat the party.  If the enemy has some for warning of what the group has that might make sense, but just every time they run into bad guys they always just happen to have stuff to defeat spell caster will get old and lame fast. IF they kill off bad guys that what they are supposed to do, right?  IMO. 

I'd say, throw the stronger enemies at your party. Sure, one good hit might KO a party member - that means they have to think tactically, not just "I run at the baddie and spam my attack button until they die". And if one does fall, someone else needs to stabilize or heal them. Just don't go TOO crazy. You want a challenge, not a death spiral.

And remember, killing them doesn't always mean they are dead.  If you TPK a party, they may just find themselves waking up in a cage near a cook pot with an opportunity for escape.   This will give them a wake up  call about the need for a few tanks. :)

And it'll be a cool story later on if they manage to escape. 

For me this depends on how experienced the players are as well as how mature.   If this is a "typical" 5e group we are going to see a lot of relatively new players, with a smattering of experienced players that might have played other editions of D&D or other TTRPGs in the same group.  There can also be a lot of CGRPG/MMORPG crossover.  If my assumptions are correct, the players might not be aware of how fragile they are.  They might not be aware that "aggro" is not a thing in D&D and that monsters and mobs with even a modicum of intelligence act accordingly.  I would communicate what you are seeing with the party in a short conversation and suggest ways to correct this scenario.  This can include swapping out a PC or two.  Players always have like 20 or more different things they want to play anyway so this might not be as onerous as imagined.  If the party listens and decides to go along the way they are, just allow the situations they get themselves into develop organically.  If they TPK due to player choices that is on them.  A party of evoker wizards and Warlocks FOREX is potentially a artillery piece, but they lack the breadth and depth to deal with many challenges that DPR can't just blow down.

This is all really great advice, thanks everyone! I will take it all on board and let you know how I fare with this week's game! 

Really appreciate all your help :)

Use an enemy with high health and low attack.