if disgaea is a big family, the first one is the edgy one, the second is the romantic one, the third one is the nerd, and the fourth one is the political connosieur.
its maybe one of the more "mature" ones (if you could have some maturity in the franchise), with the characters being all about making a coup to the government.
its a really fun game, one of the tightest disgaeas in term of gameplay, story, music, and overall fun.
here comes the obligatory friendly reminder when looking at the series: no, you don't and shouldn't have to grind to have fun.
yes, the game flounders everywhere about all the nines they could get in a character, but thats the aftergame. grinding before ending the story will only ruin the experience. but grinding after the game is over is one of the most entertaining parts of the game, its not about grind, its about optimizing grind, getting a strategy about how to develop your characters its part of the fun. its not like an rpg with hard bosses where the only exit is grinding. here the aftergame is about having that boss that invites you to grind. or to shutdown the game and go play something else.
its a well balanced game, and a total improvement from 3. and maybe the best 2d disgaea if you ignore DD2.

i would totally recommend it as your first disgaea, because is this is not your first one, i'm really surprised that you have not play it yet.