Just 4 RBP's a ten is a bit small for their current size. Now if it was temporary, till you upsized to a bigger tank they'd be fine in a 75 gallon full grown/adult. Rule of thumb one inch of fish per 1 gallon water. They can grow out over 12"s in some cases depending on diet, water quality, & natural light is a must, but not to much otherwise you'd get alot of algae prolonged direct sunlight is bad.
Filtration i use external bio mechanical Emporers 400 w/ a 402 powerhead per tank like running double filters but alot better since they do get messing feeding. Weekly cleaning glass & gravel ll filtration & 20-25% water change.
Stay away from feeding live feeders especially goldfish lfs, red meat bad for them. I'd say stick to white fish meat like frozen smelt less oil based one at a time at room temp less crap you'd won't have to clean cause it does shred or cube cut it half frozen big enouh for the size of their mouthes. I did try salmon & catfish fillets they are full of fish oil = not good for your tank.
You can use your 10 gallon for minnow or crayfish feeder tank. Me when i do get minnows a treat them with melafix incase the live feeders have something, so when i give them a monthly treat i won't take a higher chance of spreading diseases threw all my tanks. Freeze dried foods are good for them plankton, blood worms ect.
Now if you do end up with some runty/scout piranhas that depends on the fishes genes. I found some just stay smaller than others, even if in the same batch as they are young others just grow out more than the rest do. I had one that always stood out just way bigger than the rest all @ nickel size when i got them. He was bigger than a half dollar within a months time out growing the rest by atleast a inch ahead of the others. So in terms i had 11 bought at nickle size growing to quarter size @ 1 month compared to him being nickle as above too half dolar size in the same time period growth spir i guess.
Just some advice i own them myself. But in the end they're your fish you decide what you want to do. If you decide you can't keep the fish like some morons do making it bad for everybody else in the pet community out there. Don't release them into the wild. Rehome our sell them private, lfs or too someone who can provide what they need. Not saying you can't handle them just aware read up on the piranhas you'd be surprised on what you can find.
It is a battle field in here too over a whole lot of nothin. Grow up he just asked for some advice can't you read!? Get over it!