I recently beat this as well and loved it despite the horrid world map. (I also agree that it was too easy after the initial couple of hours.)
I've got access to all of the Nintendo portables and a Vita. Which other game in the series that I could play would be most similar to SMT IV?
I picked up Pokemon on release as I wanted more content in the same vein, but it just feels like an empty shell of a game compared to SMT.
Most of Pokemon's depth comes from its Player-vs-Player combat, which takes a lot of time investment before you get any payoff.
MegaTen games tend to differ wildly between installments, so it's a hard call as to what to recommend. I'd say if you can find it, go for Strange Journey.(Or if you have a PS2, Nocturne) Strange Journey is made by the same people. It's a little rougher around the edges but at the core it plays the most similarly.
Nocturne is generally considered the best core SMT game, but it's extremely unforgiving even compared to Strange Journey (which itself is a doozy) and plays quite a bit differently to IV.