2000 3.5 V6 Help

Cracked head at cyl #3 at only 37K mi. It's out of warranty so a local shop with a very good rep replaced the head. They had some problem setting timing until they found the TSB on timing belt. BUT, now it has a flashing CEL, code is mis-fire cyl #4 - lean (not the replaced head, the other side they didn't touch). Also Random Mis-fire code at times.

They reset the timing belt numerous times, swapped plug, coil,and even injector between cylinders but error stays at #4.

They're at their wits end, anyone have an idea ?

Thanks.