4 speed kits are available, I heard the 4 speed boxes are actually stronger but I have no hard data on that. I think they can just fit more clutches inside them if you build it.
It is not an electronic issue, if the solenoids are faulty it will throw a solenoid code. Mine has no 5th at the moment and 1st 2nd and 3rd aren't happy. No fault codes just slipping clutches.
Do not get a high stall converter, I tried one with mine and had to disconnect the torque lockup clutch in the end. All you will do is lose power and response. I ended up going back to the stock converter with a heavy duty lockup clutch.
PM me if you need info, not many people understand these cars and you can end up spending much more than you need to. Best option may be to do a manual conversion, at least it's adding value, or just replace it with a wrecker 4spd auto with shift kit.