Going to have fun with this one if it is a bios issue causing it. That board may have been made by MSI, but it was made for the OEM market and as a reslut at least according to here,
Can't find MSI P55M-SD40 on MSI
is up to the computer manufacturer (ibuypower in your case) to provide any bios updates if that is what it turns out to be.
As Ken said your images won't enlarge, but enough can be seen to see that half of the 8 GB is hardware reserved. That can usually be caused by one of 3 things,
1. The setting in msconfig that Uber Philf mentioned.
2. Memory remapping is not enabled in the bios. This setting is usually defaulted to enabled with the option either hidden or removed in most recent bioses. It probably isn't there in yours, but have a good look through every section in the bios just in case.
3. For whatever reason the board only supports up to 4 GB of ram, which again brings it back to a bios issue as the chipset on the board is more than capable of supporting 8-16 GB (or more).
If the memory remapping option isn't available, seeing as you just got it I would start with contacting ibuypower and seeing hat they have to say; even more so if it came with the 8 GB and you're only using 4 GB of it.