I think the 92-94 seats are a straight swop and possibly the 94-97 but these may have the side airbags,there have been reports of the 97 on series IV seats also being fitted but major modifications are required to do so.
I don't think it is that simple. Significant changes were made to the LS400 front seats for 1993 including lumbar support and vertical adjustments for the front passenger seat. I don't think the 90-92 front passenger seat wiring harness can support a 93-94 seat. If your 91 has the memory feature, it may not be able to control the 93-94 driver seat although maybe it can be done with modifications. You are safe getting 90-92 seats although you should confirm whether you can use a non-memory seat with a memory harness/ECU and vice versa if the 90-92 seats you buy are of the opposite type.