A good transmission shop would use a pressure gauge to diagnose this, but a few suggestions. Fourth places the most stress on the hydraulic system, because both clutch packs are engaged. By all means, do the usual incantations: adjust the cable, replace the K2 spring, K1 also if you're there. Change the fluid, add magic jamba juice. But the most likely cause is a worn clutch pack or seal causing an internal leak and drop in working pressure. Typically, it starts when the fluid is warm and viscosity is lower. But this will get worse in time, eventually it won't shift into 4th at all. And this can only be fixed the hard way...