New 21.5-inch Retina 4K models expected.
New MacBook Pro Does Not Support 8GB RAM (for now)
Preliminary results show that while the new MacBook Pro did recognize the entire 8GB of RAM, during actual usage, the computer appeared to be limited to 4GB:
The machine would hit an invisible wall at 4GB and act like it had run out of memory. In the larger screenshot, two instances of Parallels can be seen (both set to 2GB of RAM). The second instance, however, only occupies a small portion of RAM expected. It's unclear why this limitation exists, as Mac OS X is able to use more than 4GB on Apple's Mac Pro. As a result, MacBook and MacBook Pro owners should not expect to be able to upgrade to 8GB of RAM at this time.
iFixit has posted a disassembly guide for the new MacBook Pros including RAM replacement and will post updates about these results after further testing.
Update: More results.