According to Henri Bergius, the next version of the Midgard software will be PHP5 only:

It took us a while to get here, but Midgard is finally entering a release cycle which will drop support for PHP4. The reasons are quite clear: simplicity and speed. The whole PHP4 end of life business has caused quite a bit of discussion in the PHP community.

The group chose to go PHP5 only for speed reasons (like advancements in their Query_Builder component) and the new features that PHP5 will allow in their MidCOM component.