first, this module has been made by transcend according to the picture - PCB number is 09-2820 which they use for several DRAMs - guess offtek is rebranding this, so no blame for transcend here... checking the most likely used Celeron B800 it should be able to support 4 GB Modules...
but here it comes, according to the picture this module is using Samsung K4B1G0846D-HCH9 - which is an 1Gb Chip, which means the shown Module should have 16 Chips instead of 8... so the picture at least does not fit the part numbers... please get us a real picture of your module, this might bring us forward - as the celeron will most likely NOT fully support modules with 16 1 Gb Chips as it would be quad rank
as you can enter bios with the 4 GB module try to run it with 1066 MHz, this might work, as many of the old CPU were not able to run quadrank with 1333 MHz or above.
and nevertheless, gimme mor details - i will find out