Won't Shift into 4th
 

My car has had an issue for the past several days with missing. I changed the plugs in the car and it is running pretty good. The issue is now the car doesn't want to shift into 4th gear whether on the highway at 70 mph or cruising around town at 45 mph. Transmission is stock 4L60E. What do I need to do?

There is a squeaky sound whenever I come off the gas at speeds where it should be in 4th gear

Circle D 3000-3200 Stall Speed
No codes are thrown

Could it be that the clutches are slipping?

It has not been tuned yet, but it does have a trans cooler on it and typically doesn't run anywhere over 150 degrees. I believe it is a billet series converter.
I honestly don't know what it is but it is running about 300 or 400 rpms higher than it should on either the street 40-50mph or highway 70mph.

The converter will eventually lock up in 4th but it takes its sweet time. Any suggestions

No it hasn't been tuned. It just started doing this recently; I've had it for almost 4 years.

Is there anyway to distinguish if the lock up clutch in the converter is going out or if the lock up solenoid or valves in the trans having an issue.