How do you have those 4 wlan cards distributed on the system (main board/daughterboard)?
Might be an issue of the total power that can be delivered to the daughterboard?
Furthermore the routerboard.com website has this note for the RB564 daughterboard:
Note! You can not use more than two AR5414 chipset (e.g. R52) cards in RB/564. If you want to use three or four miniPCI cards in RB/564, you should use other chipset based ones for those extra slots, like CM9. For example, two R52 + two CM9 is OK, four CM9 is also OK, but not three or four R52.
Not having the chipset specs for the SR2 at hand, I can't really judge if this applies here...
Perhaps try to reduce the number of installed cards for testing if it gets stable then. Or put one high power (SR2) and one normal (R52) card on the mainboard and daughterboard each (to distribute power load)?
Just some thoughts...