It's safe to upgrade rb 133 and 112 to ROS 4
Sure - if your license permits. However I don't think those boards were projected to live this long! From personal experience RB133 doesn't do so good with v3.x. I would stick with 2.9.51.
and connect to an AP with 3.xx installed?
Sure you can.
Can i have some problems doing this?
Only problem I can envisage is that the RB133 and RB112 don't have what it takes to run v4.x smoothly. Some user did mention that installing only the packages you will use (system, routerboard, dhcp, hotspot for example) may help.