Biggest leak in my game: down to four.

[Matador225]

I feel I am pretty strong early on w/ a full or almost full table and my headsup play is also a strength, but I have some problems when it gets down to the last four.

If I am third in chips and have about 1.5 times the shortstack (1500 to his 1000) and the fairly tight chip leader raises 3 X BB to 600 what kind of hands should I be pushing with?

Can I push w/ 99 or do I need something a little stronger? Do I push w/ AQ or AK?

Obviously I don't want to put my stack on the line on a coinflip or worse here and the chances the chip leader folds are pretty slim. Whats your standard play here, assuming the chip leader has a fairly strong range of hands here.

Thanks for the help,
Jack