I know Reese made a 4-bolt rail system, but I've no experience with it. The 10-bolt system is more secure, and likely the reason it was more popular. Many times the installer just used 8 of the ten 1/2" bolts.
Four 5/8" grade 8 bolts into the frame, should be secure, when torqued properly. I would check them often to begin with.
Jerry