is it really truly sweet to taste, or are you assuming it is due to gravity? 4.3% ain't so bad for pils - PU (albeit Czech style) is only 4.4% I believe.
You *may* lose another point or two in the keg while lagering, but you are pretty much done.
pitching more yeast is not going to do anything at this - unless if you had access to a brewpub, you could maybe get some very active slurry and pitch a few quarts (yes I said quarts) of that, which I probably wouldn't bother with for 2-4 gravity points.
BTW - according to white labs, your yeast can have low attenuation (68-76%) so I think you're just done - next time I would use a workhorse like 830 instead - its a great all around lager yeast (I use it almost exclusively), but its especially good for Pils IMO.