Rock Band 4 and Guitar Hero Live appear to be going in two separate ways, but that's not necessarily a bad thing.
Harmonix began streaming a video showing off what the game is dubbing "Freestyle Mode," which is meant to invoke those very special feelings of wailing on the guitar with some hot solos, despite not actually knowing how to play guitar. And that's my favorite kind, because I can't play it. At all. Rock Band 4 must have been made for me, then.
Freestyle Mode allows you to take parts of songs and make them your own with solo segments that look totally sweet, drawing inspiration from drums, where you could already do this previously. Except instead of just slapping at whatever part of the drums you want to make noise and hope that you rack up a good combo, the guitar portion is meant to add substance and depth to the songs.
For this reason you can't play exactly what you want, down to the letter, but a series of prescribed note patterns that were created especially for each song to sync with the music itself. Think back to Amplitude, another property from Harmonix that's currently being remade. You could "remix" tracks, but no matter how you behaved or what direction you went in while "remixing," the track was still aurally pleasing. It's the same philosophy here in Freestyle mode.
When the note path changes over to Freestyle mode, you can implement a series of strums and note combinations, which you can learn or just guess at. It doesn't really make a difference how you play so long as you do something when the spotlight's turned on you. According to Harmonix there are 25 different note combinations to work with, depending on how you hold the guitar, how you strum, or which frets you decide to work with. This was part of the focus for Harmonix when creating Rock Band 4, remaining new and innovative while sticking to familiar convention.
Of course, the guitar parts aren't getting all the love. Those who like to make up their own lyrics when singing (like yours truly) will find the vocals' Exultation Mode to their liking, where you can croon your own stuff if you stay in tune. I do that anyway, so being encouraged to sing whatever I want sounds right up my alley.
Rock Band 4 looks like it's poised to cater to several different styles of play, and that's an exciting prospect. The game is due out this October, and will allow you to play with the songs you've already downloaded as DLC and those you see in the Rock Band Music Store. Now they just need to add the entirety of Queens of the Stone Age's discography, and I'll be set.