I have java moss in my 20, and my RCS relish it. I started out with six, and they bred in my tank. I've counted a total of ten thus far, that I can see. There are probably many more.
They also like to hide amid the plastic fronds of fry mats (floating at the surface with "grass" side down).
The main problem is trying not to siphon them out during water changes. I always check the buckets before disposing of the water -- and net any shrimp back into the tank. They don't always glow bright opaque red. Sometimes, they're more transparent and not that easy to see until they move.
Mine also like to climb all over the java ferns and anubias. And the rocks, the mossballs and everything else they can cling to, including the prefilter sponge. It's fun watching them propel themselves through open water when moving from one place to another. They kind of float-swim.
What do you feed yours? And is your tank established? As you've been advised, no copper -- that's not good for them.
It could be that what you thought were dead shrimp was really shed skin. If you consistenly count only four, then it's a safe bet the others have either died or are hiding amid the decor. You might be surprised to suddenly find the population increased -- meaning they've either bred or the ones you thought were dead were just out of sight for awhile.
Good luck with them -- once they get established, you'll have lots of them...
20 gal. high: planted; 8 white cloud minnows, 10 RCS, 2 blue shrimp, several snails; AC50, Azoo air. 65 gal: planted; 7 rosy barbs, 6 glofish,, 2 zebra danios, 6 rosy red (fathead) minnows, 3 dojo loaches, several snails; AC110 x 2.