Sorry my friend, Do you have any other codes? If the engine is running rough that would prevent the fourth gear shift (knocks sensors, etc) But otherwise there is probably some debris in the valve body assembly that is preventing the shift. That means rebuild time. Dont bother wasting money on a flush, it will make it worse. When you dropped the pan was there metal debris? Almost everyone with your symptons ended up with a new tranny. The dealers will rip you to a tune of $4k to $5 . An independent will charge around $3000. If you know of a good independent transmission shop that has good references, I suggest you have them look at it. There are two speed sensors on top of the tranny that you could try switching and see if your symptons change, but that is probably a last hope...Keep us filled in. Also do a search on this forum for RX300 transmission problems and you will see a lot of discussion.