I currently have two stallions and four mares I'm training so I can breed them. How should I breed them? Should I breed all of them? How do I prevent inbreeding down the line? Should I get more and train them? 

TIA

I have one mare for each stallion and only breed once for myself when they're 20 for this reason. I may do a backup breeding when they're 17. But this foal is for sale and if no one pays it, then it's put up for deletion.

You could split them into two dividers of one stallion and two mares. When you need more mates, buy them from the Store or foals from other breeders.

Link to breeding advice: https://horsephenomena.com/general_help.php#animals

Alright, thank you! I bought two more stallions and bought some extra energy so they can catch up to the rest of my horses :)

Easiest way to keep from inbreeding? Use a family tree chart. 

Yeah, I just keep one male for one female, and I only breed one time.