It only shows up 4d8 in the spell. Base on this thread, it should be at least 4d8+dex.

 The spell says

The target takes 4d8 lightning damage on a hit, or half as much damage on a miss, instead of the weapon’s normal damage.

So the spell wouldn't add dex to the damage

If you click the link, it provides you to a Twitter link to Jeremy Crawford's Twitter post.

It has the following:

 The lightning arrow spell replaces the weapon's normal damage. Roll the damage in the spell, and apply any modifiers that apply to your damage and that aren't specific to the weapon you're using. #DnD

Quote from Frozen_Fruit >>

If you click the link, it provides you to a Twitter link to Jeremy Crawford's Twitter post.

It has the following:

 The lightning arrow spell replaces the weapon's normal damage. Roll the damage in the spell, and apply any modifiers that apply to your damage and that aren't specific to the weapon you're using. #DnD

 

 
You're misinterpreting that.

"That aren't specific to the weapon you're using" means you don't roll normal weapon damage (such as the dex bonus). It's worded that way because other features might apply modifiers to the roll which could be added.

Ok, I guess did. Just curious, what other features might apply the modifiers?

Pretty much any extra damage things. Protector/Fallen Aasimar can do extra damage equal to character level, Fey Wanderer Rangers get 1d6 psychic damage on weapon attacks with ability to also deal another 3d6 while Horizon Walker Rangers do 1d8 force damage, Hunter Rangers taking Colossus Slayer can also deal extra 1d8 damage on a weapon attack, Monster Slayer rangers get to deal extra 1d6 damage on a weapon attack, Swarmkeeper rangers can add 1d6 damage to a weapon attack. Then there's multiclassing, like Draconic Bloodline Sorcerers (Blue/Bronze) adding their Cha mod, Clerics with Divine Smite add 1d8, Rogues get Sneak Attack, Paladins can add Smite... And so on.

The spell replaces the base weapon damage of 1d?+str/dex with 4d8. But everything else you add, you still get to add. This is why the feature is worded the way it is.