No way is SCV4 overrated. Everyone's talked about the atmosphere and music which goes without saying. It's one of the few 2D games that I've always found to be very immersive. You get a sense of how dark, foreboding and grimy the world is. The game is a bit easy but I don't think it's easier than Rondo.
But let's talk turkey. SCV4 is truly deserving of the "Super" in the title because the game is EPIC in a way not many other CV games are. Just look at the map screen and the terrain you're traversing. It's a long time before you even get to the castle itself. It's almost like they wanted to make it seem like a reward just getting to the castle at all and give you a sense of how big Dracula's estate is by making it such a long time before you get there.
THe game does a great job of slowly building up the intensity. By the time you get to Stage V, you've already been through quite a bit and the atmosphere of that stage, from the beginning to the end when you finally walk right up to death's door, is INTENSE! They then dial it down a little for a stage or two, but build it back up and by the time you get to Stage VIII, your nerves could be shot. SCV4 is a high water mark of the series in terms of pacing.
There are a lot of "Holy Shit!" moments in this game, from the first time you play through the Rotating Dungeon, Stage V as a whole, the chandeliers in Stage VI, the Bat Bridge in Stage B, the Room Of Close Associates, and finally the candles lighting themselves after the (incredible) Death fight, and that creepy-ass music that accompanies it. There are more but these were always my favorites and thinking about it now just makes me realize how amazing the game really is.
As far as the 8-way whip is concerned, I think that was a last minute addition to the game. If you look at the way the game was designed, enemy placement, etc, the whip breaks a lot of it, especially in the earlier stages, which makes me think they put that in after most of the game was done.
I played this when it came out, and I have to say, it was amazing to see this game in action in big beautiful 16-bit after playing the earlier games. The game just seemed HUGE at the time, even compared to CV3. They just seemed to have so many great ideas, some of which took advantage of the new hardware, which was par for the course for Konami back then. Every game seemed better than the last and for a 1st gen 16-bit game, this one has aged really well like folks have said.
TL;DR SCV4 isn't overrated in the least, it deserves the accolades it gets even if just because of the atmosphere, pacing, and design.