As a sad part of SNK's history, Metal Slug 4 was under development during SNK's bankruptcy :-(
Perhaps as a result of this, MS4 doesn't manage to bring as many fresh ideas to the table as previous titles, and is forced to reuse more graphical assets from former games. But without a doubt, Mega Enterprise / Noise Factory (who actually did the music for the previous game) should still be credited with an incredible effort on keeping the Metal Slug series going in the face of uncertainty.