How many cherry red shrimp can I have with my betta in my 4gal tank

You can fit a good handful but I would start with like 10. Be warned, not all bettas get along with shrimp and a lot of bettas will compulsively hunt them down one by one. If the tank is planted, that will help the shrimp hide. Shrimplets will certainly be eaten, but again, a planted tank will help.
Some people like to test their bettas with ghost shrimp to see if they will leave the shrimp alone because ghosties are cheaper.

Ya I'm testing mine with one cherry and so far a weeks gone by and he's still alive the betta is still small bought it as a baby so going he gets head to then so he won't eat them.

Sounds good. :) Are you ordering the shrimp or getting them locally? If you are ordering them, I would get a few extra to account for possible DOAs and any deaths during acclimation. If you are getting them locally, then its easier to just go back to the store and get more if anything happens. :)

I agree with angel about the betta snack but considering your experience so far I would say you are good. Rule of thumb is 10 shrimp per gallon so you could have more shrimp than you want probably lol

I'm not sure where I will get them because a lfs sells then for 3$ a piece and online I can get 10 for $ 13

Yeah, $3 is pretty steep. I got mine from jetajockey. A few other members on here sell them periodically too, so you are open to ordering online I suggest looking at the classifieds section on here. :)
I would just order them online and you would pay less (including shipping) than that lfs is charging for them. Just get a few more of them. With the betta in the tank, its likely you won't get a ton of surviving shrimplets, so if you order online you could start with like 20. Besides, you do end up with lots of extra shrimp in your tank, you can always trade them in at your lfs. I trade my rcs into my lfs for store credit because they don't have a good, cheap supplier. :)

Ok so I think I jinxed myself when I said nothing has happens yet because I came home and found my little guy in my filter:-( what can I do to prevent this from happening its a aqueon evolve and has some sort of a built in sump

What I would give to be able to have cherry shrimp :(

Put a pre filter sponge over your intake tube or nylons to keep the shrimp from being sucked up.

You meant the shrimp was in the filter right, not the betta? Thats what I assumed you meant, but I am just making sure. :)
Like Discus apisto said, you can put a pre-filter over the intake so it doesn't suck anything up.

Ok I'll test it out and let you know what happens and thanks for all the help and yes I meant the shrimp