A 4x8 machine made from MDF is not a good idea. Even 3x4 is on the large size for an MDF machine. Gantry sides made of just 3/4MDF will not be strong enough. At least 2 layers would be needed, or a torsion box type structure.
If you haven't read it, read Joe's thread. It's probably the best example of an MDF machine on this entire site. Joe's CNC Model 2006
Do you have any specs on those motors? I couldn't find any info.