Sort of. The 4cyl in '89 had the A340F transmission. The 6cyl had the A340H. This guy swapped an A340f into his A340H truck, and I'm pretty sure I did the reverse on my old '86 although I didn't know it at the time.
Your biggest hurdle for you will be the bellhousing - the transmission from the 4cyl truck will not have a bellhousing that fits your V6 motor. I think the bellhousing from your truck will bolt up to the donor transmission, but I'd check on that before pulling the trigger. If you can find a workable bellhousing, there shouldn't be any other big hurdles.
Can you not find an A340H? Seems like V6/automatic trucks are all over the Pick and Pull yards down here in the valley. Have you checked inventories at car-part.com?