1990 740GL is having problems engaging into 4th gear and it pops out when on freeway. Is anyone formiliar with this same issue? The arrow on the dash is not lit up so I'm assuming the od is on.

 

When was the fluid changed last? Start by changing the relay, it's the problem most often.

 

this is an automatic or manual with overdrive?

if its an automatic, does the OD light toggle when you push the button on the shifter while in drive? if the light reliably toggles, then the relay is probably OK. the light on means OD is disabled. when the light is off, then the transmission will shift to OD(4th) when it feels it should, based on load and rpm.

if the ligth toggles with the button, but it doesn't stay in 4th when cruising with light throttle at freeway speeds then the problem is likely inside the transmission, low fluid, or dirt particles or plugged up hydraulic control stuff. As Lev asks, when was the ATF last changed? is the ATF level low ?