Valis IV is the final PC Engine chapter (though the first was released subsequently) in a series that prides itself on tough Castlevania-style action and cute-girl cinematics. It doesn't feature the same caliber of level design as III's most ingeniously constructed stages, nor does it star characters as cool as III's, and its music is disappointing. But it definitely sports the finest graphics of the PCE Valis bunch, and it's long and consistently challenging, whereas its brethren are either short and easy (the first and second episodes) or wildly erratic (III). It should be no surprise to Valis veterans that the controls aren't quite perfect, but with practice and careful observation of enemy patterns, you can indeed devise plans to complete each stage in perfect fashion, making the whole experience very rewarding--if frustrating at times. In this regard, the game and its immediate predecessor are kind of like the action-platformer equivalents of R-Type.
The stages look good and feature cool, robust enemies; and for the most part, they challenge you consistently rather than alternating between easy and incredibly difficult segments like III's do.
Some of IV's toughest challenges, like the fourth-stage cathedral ascent, can be tedious and irritating without feeling as rewarding or as cleverly conceived as III's most demanding trials. The level-draining fetuses that appear towards the end make real nuisances of themselves.
There are lots of bosses, too. Battles with them can be long and exhausting. You'll face a few unexpected opponents before all is said and done.