I have got a close group of 4 really hot girls who are my friends. I figure they are a untapped resource that I could use as social-proof.

However, I don’t really flirt with them as I don’t see them as sexual beings…we’re all distinctly friends now (I think 1 of them used to fancy me but I wasn’t really interested). I've known most of them long before I became interested in PUA and have generally shied away from letting them see me/know about me dating/picking up girls etc

So anyway, they don’t really flirt with me. Does this ruin the whole ‘preselection’ thing or does the fact that I will still be seen talking to 4 really hot girls still work in my favour, even if they aren’t visibly flirting with me?

Any social proof is always better than none. Whether these girls flirt with you or not doesn't really matter. What matters is what happens inside the head of those watching you. They don't know who your friends are and what kind of relationship you have. You can as well be a polygamist, and they are all your wives.

The thing is that they see that people enjoy being with you, which is enough to make them curious. They just have to find out "is that guy single or not?". And they will unconsciously start seeing your friends as competition, and will therefore try harder to get you. If you give them just enough attention and IOIs to make them feel special, they will think they have a chance, that you will choose them. The funny thing is that you can show the same interest to several girls at once, and they will all start competing about you, because they all think they have a chance and that they are better than the others. You can take it to its extreme, and have girls all around you screaming hysterically for your attention, and at the end of the night you can just pick anyone you want to have sex with. That was how I met my girlfriend, haha.

Girls are girls man. If you're seen with 4 girls (regardless of how you know each other) other girls are gonna probably get curious.