NOTE: This guide is for those who aren't very good at playing the instruments above MEDIUM level and are reasonably good at singing.
I'm going for this achievement on VOCALS only, since I'm no good at guitar and drums anywhere above MEDIUM.
If you're not completely tone-deaf, this is perfectly doable on vocals.
Here are some tips.
1) get familiar with each song first
2) words don't matter at all, if you have trouble with some phrases, just go nah-nah-nah or ah-ah-ah... actually singing the actual words might make it difficult even if you know them by heart. You see, some sounds (voiceless consonants - like p, t, k, f, s, sh, th, ts...) do not use your vocal cords and therefore the game can't read them as tones. That's why it's better just to sing in vowels, ahs, oohs etc.
3) hold the long tones all the way until the end. Actually it is a good idea to hold all tones a little longer than seems necessary. If there is a series or a few shorter notes, try to link them together. Also, if you're experiencing some lag from the TV, make sure you start singing a tiny bit earlier.
4) RB4 songs have freestyle vocals, which means that instead of some difficult phrases you can just produce a long tone (if it fits the key) instead of an elaborate melody which will always cause you to lose your multiplier. This saved my ass on so many songs!
5) If you struggle with a song, ask a friend to help you out. Just ask them to use a chatting mic connected to the controller. Both of you can sing on SOLO and in this way even if neither of you sing perfectly, the game will just put both performances together and if at least one of you is singing the right tones, it will registrer the phrases as perfect. This saved me on CENTURIES (Fall Out Boy) where there seems to be no time to breathe! With two people, it's a breeze!
This solution should help you get:
Rock Band Fan
Earn 5 Stars on Medium (3 Stars on Hard/Expert) on any 30 Rock Band 4 disc songs in Quickplay.
Rock Band Master
Earn 5 Stars on every disc song in Rock Band 4 on Hard in Quickplay mode.
Rock Band Immortal
Earn 5 Stars on every disc song in Rock Band 4 on Expert in Quickplay mode.
Living Room Legend
Earn Gold Stars on any thirty Rock Band 4 disc songs in Quickplay mode.