Rain-Slick 4 is an improvement on Rain-Slick 3 in almost every way. Once again, I played through the game on Insane difficulty. Due to the new monster ally mechanic, party member roles are not as rigid, and many battles require a different approach than using the same team composition over and over. While the overall challenge is slightly less than 3, it felt more rewarding to clear a hard encounter through modifying my strategy rather than just retrying and hoping my tank takes more hits; not to mention, there are still plenty of formidable opponents in the optional content.
Aesthetically, the lush landscapes and eccentric creatures of Underhell are far more delightful to behold compared to the streets and buildings of New Arcadia. Along with that, significant upgrades have been given to the music and sound effects, including a couple of really stand-out boss themes, which is excellent because boss themes are the most important aspect in judging a JRPG's quality.
My only criticisms are that, for most of the game, the focus switches between two separate parties with their own monster groups. Equipped items are not shared and money is rather scarce, so having to re-optimize after each swap can be quite irritating. The story is also barely-existent, essentially serving as "an event happened in the previous game and now you have to kill these things and then a final boss so the series can end." There are a few twists, but they are hard to care about because the characters are primarily vehicles for jokes rather than empathy. This was expected, as Penny Arcade has primarily been about comedy, but again it tends to be hit-and-miss. Personally, while many of the outlandish enemy designs and descriptions genuinely entertained me, the dialogue has a prevalence of snark, walls of text, CAPITALIZED SENTENCES WITH NO PUNCTUATION, "hitting stuff is funny," and "we all hate everything" that I can definitely see irritating certain audiences.
Still, this is Zeboyd's best game to date, and indeed, one of the most well-refined JRPGs I've played. An exceptionally good deal at $5 for ~15 hours of game time, and an absolute must for those who would rather think than grind.