Best how is the first question that comes to mind? Best lock, best action, best looking, best steel? Best can mean so many diffferent things. For the price you are budgetting you can find a great knife but best?
Have you considered a Cold Steel Voyager or Vaquero w/a 4" AUS8 steel blade? The Recon 1 might do the trick also in a 4" blade.
But if you want to go premium you could find a Emerson Sark in 154CM steel for a blade with a blade slightly less in length than you are looking for, for just over what your limit in spending is or get one of the "Hard Ware" line of Emerson knives for around $70 plus or minus.
You didn't specify lock type so I have suggested three different locking mechanisms.