I'm a huge fan of SCGMD3. I've perfected all the songs in that game multiple times. Nearly every problem that the second game had, the third one fixed, and there was so much more to offer that seemed to really enhance the feel of the game in general.
This, however, was a disappointment.
You've got a knack for programming, and you really know how to make a game look professional, but there are basic aspects of gameplay here that felt rushed, like nothing was specially planned beforehand, and the whole game suffered as a result.
The first thing I noticed was the downgraded frame rate. I know it's not lag on my computer. The arrows and letters seem to blur and chop across the screen, and that feels 'off' in a rhythm-based game. I set SCGMD3 and SCGMD4 side-by-side, and the arrows in #3 move much more smoothly than in this game, and this minor detail contributes to other bigger issues.
The eye candy of this game discourages focus. The running stick animations are clever and well made, but they're very distracting until you get used to them, which takes time. The human eye naturally focuses on whatever is moving most on the screen, and when a guitarist is dashing at light speed while surfing and flashing and jumping around, he's demanding more attention than the game you're playing is. I also get irritated by the popping animation the arrows make when you hit them as they cross the bar, because that animation covers up the next arrows in the same row. I turned off this feature and it was even worse, because there was barely any indication that I was hitting a key correctly. Previous games managed to find a safe middle-ground.
Over time, I discovered that many of the songs are full of notes that don't quite match the song itself. Many notes arrive at the bar a bit too soon, and many notes arrive a bit too late. This is this game's most significant glaring issue. When it matters that you're supposed to be able to get a Perfect Score on every song, that feat no longer seems reasonable if you can't hit the notes correctly and have it be registered. I never had problems with this in SCGMD3. All the notes were carefully placed and the game was developed to be forgiving in the right ways. I never worried that the game's programming would cheat me out of hitting all the notes in a long string of up-arrows. I knew it was always my fault I didn't play a song perfectly.
Even if all the notes in this game were properly synced and the programming was done as cleanly as the third game, more problems still remain. This game's difficulty is as 'bipolar' as I've ever seen. The Amateur songs are easy to the point where I've phased out in the middle of a song from boredom and kept hoping it would end. The Pro songs are the exact opposite. The messes of notes that pop onto the screen are often so excessive and disorganized that it's instantly overwhelming. It doesn't help that all of the notes in Pro fly across the screen faster, especially when the game's frame rate is less than ideal. One of the pluses of SCGMD3 is that you always had plenty of time to react to the notes ahead of you. It was never disorienting--it felt fair. I don't know if you made the notes go by faster because Guitar Hero uses this gimmick, but the difference is that GH has an excellent frame rate and the flow of the game remains intact.
Lastly, the "boss battles" are completely unnecessary. They seem like they're only there to make the game last longer. It's like they utilize all the game's other issues to supposedly increase a song's difficulty in the form of some comedy stunt, and yet they're so embarrassingly easy to beat that you're sitting there for two full minutes just waiting for the song to end and you earn nothing worthwhile as a result.
I understand that you were trying to make this a 'cool' sequel with creative, adventurous ideas (and in some ways, you succeeded), but those ideas were sadly expressed with mediocre design and unpolished gameplay.