1. As I am targeting for INSEAD, LBA, Oxford, and Cambridge, not sure if this score would help. ( For IMD, I have been wait-listed )
A higher score ALWAYS helps!

2. Will the fifth attempt with not much improvement in my score be a major weakness?

Not at all! Schools don't really care how many times you take the GMAT, esp since you have been getting better every time!

3. Is it better to go ahead with the application process as I for INSEAD and Cambridge is in early October or should I retake my GMAT and improve my score

That's up to you. Do realize that at SOME point, you will reach your maximum score. Not everyone gets a 720, and at some time it's time to just go and apply with whatever score you have. YES, this will make INSEAD and LBS stretches. But do you want to try again and wait another year? It might be better just to adjust your school list, to add some lower-ranked schools, and apply with the score you have.

Thoughts?

Well you can keep INSEAD on, as it is your dream school, but realistically your chances are pretty slim. You might be better off finding another school at where your chances are better - like HEC, or IESE, and it's not like these schools will be super-easy either!