Thanks for the tips! Hoping to find another 40g breeder in the future and set it up as a planted (shrimp) tank, but I'm limited to the 4g to try it out for now.10 Would be a good start for a 4 gallon. It will be harder to keep them in small tanks like that. The general rule is 2-3 per gallon, but I have seen people do 10 per gallon many times. IE 100 in a 10G tank.
The main idea being that the more total water your working with the more of a buffer you have. (The more time you have to fix something if it goes wrong, and the harder it will be for it to go wrong in the first place!)
I would suggest any of the NeoCaridina Species (except maybe the yellows or the new fancy blue ones which are more picky with parameters) and avoid over feeding as that will be the number one way things will go south with new shrimp keepers!
Cheers and welcome to Shrimping!