I bought Rock Band 4 digitally. Found that my old guitars don't work that great for it, so I was looking at buying 1 new one. The only way for me to do it back then was to buy Rock Band 4 again with a guitar. I already had Rock Band 4, I just wanted a guitar!
I see they are finally selling the instruments by themselves, but with the new partnership and people saying how the original RB4 instruments felt really cheap, I'm holding out for the new instruments.