Help It won't shift into 4th gear

[GP]

Hi All,

I have a 94 SL500 that suddenly wouldn't shift into 4th gear. I checked the ATF and it was fine. I tried driving it around waiting for it to shift automatically, but it just stayed in 3rd.

I even tried while driving say 50mph to shift into N (neutral) then back to D (drive) it would shift to 4th for a moment then once I apply gas it would upshift to 3rd.

I don't hear any noises or signs of transmission problems, all other gears 1-3 and reverse work.

I even had a friend who works on mercedes check the vacuum near the transmission pan, no leaks in the vacuum.

Also i made sure the snow traction wasn't causing it.

We even went to the extent of draining the transmission plan of ATF and putting new ATF.

Still no luck!!!

So any info would be appreciated.

Thanks
GP