After driving the car a good bit for a few days, I noticed the problems I'm having shifting into 4th and lockup are not present while under load/going up hills or when cold.

While the car is still cold, it will shift into 4th at 45, but after warm, I'm lucky if it will go into 4th at all, even at speeds up to 60 sometimes, but if I have some load on it, such as going up a hill it will kick into 4th and work fine until I'm back on flat or downhill.

What the heck? I didn't realize this before, but I've had to drive it a lot lately.

Transmission fluid is fine.

I have been doing so much work on this thing lately. Just did the fuel system, which included rebuilding the TB, replacing the fuel pump+strainer, filter and even the relay.

It's a 700R4.