It sounds like you have spent a good bit on upgrades so far, but you still have two big ticket items left...fork and wheelset. You may want to at least consider saving up for a different bike before dropping more money on the 4300. Of course, you can always take the upgrades over to a new frame, but your choices will be limited.

To answer your question, I would go for a new fork without a doubt. The wheelset and crank/bb are heavy, but also reliable. A good fork is going to be much lighter and better performing than what you are currently running. Also need to take your budget into account...if you have less to spend, then replacing the crank may be a better option.