I've played EBF IV on Newgrounds and it was one of my favorite games, also considering that I've played through I~IV each on Epic Difficulty (although, the first of the series I have not succeeded in completing on Epic Difficulty). Seeing it released on Steam and buying it on the early release got me going with hours of gameplay.
Either way, it is different from the Flash Version, with Extra content and New Game+ and New Game++. Extra Content includes such as the Battle Mountain area which requires more of a strategic approach than the grinding approach. New weapons and armor to help customize to whatever best fitting for you through gameplay challenges.
Difficulty-wise, you can change the difficulty at any time necessary (although, one should be playing on Epic Difficulty); for those who need a challenge or are having trouble with a slime. You should be using strategies you know.
Gameplay-time, you can expect to look forward to of around 25 hours to 30 hours of gameplay for your first playthrough (and running on Epic Difficulty while gathering a majority of what you can).
It provides references to other material while adding its own hint of humor.
If you're still on the line of buying it or not, consider trying the Flash version for a few moments. If you do enjoy it and wish to support the creator, Matt (and HFX), then you may buy it as so. If you have travelled quite a long distance in the adventure, there is a way to transfer the saves from Flash to Steam.
Overall, if you like turn-based jRPGs with wonderful quality graphics and a different plot than most, I'd recommend it.