you still can use the 4 prongs ballast... if you see the ballast will notice it got some slots. 4 prongs ballast plugs got a key/stud to be sure will be installed allways on same indexing, to be sure the high resistance and low resistance paths will be propperly conected to source whichever have to source.

On a front view, if you fit the ballast with notches/slots toward to the driver side, use the top prongs to the new harness.

Other wise, just get a single ballast ( 2 prongs ).