I want an HB8. On a group hangout we were introduced to a 4 who is into me. The 4 may be good as a friend but I have absolutely no other interest in her.

The HB8 saw me exchanging messages with the 4 and started asking questions. I simply cut her short with a "don't be nosey".

How do I pawn the 4? On one hand, I may use her to prey on the 8's insecurity and jealosy, making her believe I am no longer intersted and she is about to lose me for another girl, wich is PUA 101. But on the other hand, what message am I passing if I show interest in a 4??? Wouldn't I be better off DHV'ing with a "yeah, sure, she's pretty funny and all but way under my league"

Don't say "she's under my league", never diminish people like that. Instead say "she seems to be into me, but I don't think she's my type". Then add "but she is quite nice though" to play on the 8's insecurity.

Your attitude towards the 4 should be vague and slightly avoiding. If she's asking you "can we hang out together some day" you can answer "I'm quite busy right now, but maybe next week or something". If she doesn't get the hint after a couple of such replies, you can either be very brusque and say "I'm not interested in you" (not recommended), or make her loose interest in you by being extremely needy and clingy - show her too much interest, text her all the time, call her several times a day and so on. It won't take long until she is fed up with you.