No, Season 4 and 5 are BOTH overrated as hell to me! While they both used to get a lot of flack, many people in the past 5 years have compared them favorably to the first 3 seasons, and I honestly can't agree with that. Season 4 is OK, I guess, and I will admit the first half is actually pretty damn good (outside of Good Neighbors and Funny Pants), but the second half has very few good episodes and a lot of meh and just plain bad episodes. Season 5 in my eyes is still a bad season, though I will be fair and admit that it is still better than 6 and 7. 5 had a lot of nasty episodes (Fungus Among Us and To Love a Patty), a few squid abuse episodes (Boat Smarts, Good Ol' Whatshisname, and Breath of Fresh Squidward), and of course the infamous TV Movie (Atlantis Squarepantis). While I do applaud the season for attempting to be more experimental, a lot of it just isn't fun to watch at all IMHO, even if it wasn't quite as bad as what would come in the next 2 seasons.
While both seasons do indeed have their great episodes (such as Fear of a Krabby Patty, Skill Crane, Dunces and Dragons, Kristy Towers, Best Frenemies, Patrick Smartpants, Krabs a la Mode, Friend or Foe, Sing a Song of Patrick, and Roller Cowards, just to name some off the top of my head), I think people vastly overpraise them these days. I do understand how some people might've eased up to them over time due to how bad Seasons 6 and 7 were, as well as the hate getting so bad for some people that they simply got over all of it, but in general I think they get far too much praise nowadays regardless.
So no, I would not consider them underrated. Quite the opposite in fact. I honestly think seasons 9, 10, and maybe even possibly 11 are better than 4 and 5.