the intel atom processors support max. 2gb per memory slot and max 2 memory slots, which sums up to 4gb.
I believe that your nas has only one slot so the max supported ram is 2gb.
This is a hardware limitation that can not be overcome by a firmware update.
I upgraded my ts-639 to 2gb and it is working just fine. This would also give you at least some headroom for vbox.
TS-639 Pro | Version 4.0.1 | 5x 3TB WDC WD30EFRX RAID5 + 1x 320GB WDC WD3200AAKS
LianLi EX-50 with 3x 2TB SAMSUNG HD203WI + 2x 2TB SAMSUNG HD204UI in RAID5 as Backup drive
LianLi EX-503 with 5x 2TB SAMSUNG HD204UI in RAID5 as Backup drive
TS-109 + TS-109 Pro II | Version 3.3.2 Build 0918
HDI Dune BD Prime 3.0, Mede8er MED600X3D, SqueezeBox Duet, Netgear GS608 v2 8 Port GBit Switch