"...got 4 new headlights that all turn on with the bright switch..."

Good.

"...lowbeam the left headlight goes out..."

Could be a bad bulb, but not likely if they're all new.

Double check the in-line connectors and the sockets that plug onto the bulbs. Look carefully where the wire connects to the in-socket (or in connector) lug. Those are usually crimpled, and sometimes corrosion eats up the wire.

With the low beams on (and the dead bulb dead) chase backwards through the wiring using a meter or test light.

And finally -- as I recall, the low beams (but not the high beams) go through the famed "bulb out sensor." That sensor has a spotty track record -- sometimes it's the cause of a bulb out (I've had several over the years). You can sometimes open it and find a bad connection or broken wire and repair it.

It's worth your while to find one from a junk or borrow one for diagnostic purposes from a good car.
--
Don Foster (near Cape Cod, MA)