If your opinion of the Quad and its cohesion is correct, then Pakistan's choice is already made. Suppose the US-India relationship really does develop into an alliance and America sells F-35s to India. Is there some law that states those F-35s can only threaten China or would Pakistan also feel concerned by such a development? If this does come to pass, where do you suppose Pakistan would get its own stealth fighters?
Pakistan - China relationship is to the benefit of both countries - this is not even a discussion at this point. China is a neighboring country and Pakistan has benefited from its help on many counts. China is also giving CPEC to Pakistan. So HOW can
one expect Pakistan to betray China? Out of question. Chinese members should not have second thoughts about this matter. Take cues from developments on the ground. And not from emotions of any member.
While Pakistan and China can collectively corner and bully India in theory, India is a big country as well and it cannot be undone and/or destroyed as per our mutual wish here on PDF. WE will also have a QUAD problem to deal with in the near future - a problem to which there is no practical counter.
QUAD is slowly but surely becoming a serious thing. Indian Navy receive meaningful INTEL about movement of Chinese forces in the Arabian Sea already. And meetings between QUAD members are more frequent then before.
It is easy for USA to exploit rivalries between distant powers to its advantage - understand this. Its reach is GLOBAL and its armed forces are second-to-none.
Now there is Pakistan - USA angle to the game as well. As much as some members might want to talk against USA here on PDF - some understand the bigger picture. Pakistani economic realities are such that WE have little room to maneuver and CPEC is the only relief in sight but not sufficient in itself (Pakistani population is growing at a rapid pace as well). USA is largest EXPORT destination of Pakistan - a major contributor to Pakistani economy in this capacity.
While Pakistan will honor its friendship with China, antagonizing USA is not a realistic route either. Pakistan is already finding India a difficult neighbor, imagine enmity of USA in the mix. Pakistani economy will be SMOKED and a lot more would diminish for my country.
I am just being realistic. Understand this.
Best course for Pakistan is to be the VOICE of REASON and INTEGRATION in the world. The integration part include CPEC as well as any other form of integration.