I think it depends on what hero you you’re pairing the troops with. For example, I got two 4* greens at the same time when I got my first 4* troops. One seemed better at defense, which would help a hero with weaker defense, and one had a bit of a mana speed increase. Since I was still using Skittleskull at the time, I used the troop that gave him/her faster mana, since his mana speed is slow. However, if I had had a green that had fast mana but was a bit of a glass cannon, I might have used the other troop that was better at defense.
Either way, keep both 4* troops, because eventually you’ll probably want to double up greens for certain Titan fights, or for when you need the benefits of your 2nd 4* troop!