If you don't need variations (alot of sites don't) then stick with 3.5.11 if selling and running a business is your priority.
If you're a developer or very confident in dealing with any issues that may arise then and only then use 4.X.
We still use 3.5.X for ourselves and our clients.
Alot of new people chose 4.X as their first X-cart. It's ecommerce high season just now, you really can't afford to be messing around during christmas.
Business first, toys later
