Static IP assignment and statically setting the IAP VC IP address should not affect this behavior.
Just a few items to check:
-What version of SW is running on your IAP-105s? The current version is 6.1.3.4_3.1.0.1.
-Are all the IAPs in the same subnet or are they in separate subnets, hence, 4 separate VCs?
-If 4 separate VCs Layer 3 roaming needs to be configured.
-What wireless clients (OS) and NIC models and version of drivers? Are they the original drivers included in the OS or have they been updated to the lates version of driver? In some cases, old drivers cause the client to be "sticky" in that they cling to the AP they are associated to even if a better option is available.
-If running the current drivers some drivers have a "roaming agressiveness" setting that can help roaming behavior.
-You can, but should not have to, turn off some of the lower data rates which may encourage the client to roam.
If this does not help you may want to open a case with our TAC.