Yes, 4 long and 4 short is rear fuel supply system. This likely indicates that the O2 sensor voltage is out of range. If the O2 sensors on your 1991 are original the error is most likely due to a failing sensor; but, it could also be due to dirty injectors or other things in a descending order of probabilities. You can do a reset on the ECU by removing the ECU keep alive fuse to clear the error code from the ECU and then drive around to see if the error code re appears. That would confirm that it is a persistent problem.
If you have a good quality high input impedance voltmeter, there are measurements that you can do to determine whether your sensor is getting lazy or has actually died (unlikely since the error code does not seem to persist - yet). It is unlikely that you have a general fuel system problem (low fuel pressure due to FPR / pump / filter problems) because you don't have a front fuel supply error. I am not a fan of diagnosis by parts replacement. However, in this case if the O2 sensors on your 1991 are original, given their age just replacing them to see if that resolves the problem would not be a bad plan. Given the age of your car, you might want to plan for replacement of both the front and back because if the back has failed the front's number may be up soon.
Replacement Denso or NTK O2 sensors from Rock Auto are fairly cheap (<$40). Replacement is not technically difficult; but, can be physically difficult because of the location (particularly the front) in the exhaust manifold and the fact that after 32 years extraction may be a challenge because of corrosion. Extraction may require high heat (torch) or other brute force methods that you might want to leave to somebody else (much easier to do this on a lift).
Similarly, removing the injectors and sending them off to somebody like RCinjection or Witch Hunter Fuel Injection to have them cleaned and flow tested would never be a bad thing on a 31 year old car. If you do that, plan for new injector seals since the old ones likely will not reseal.
Finally, if you search on Prime for service manual you can find links that will allow you to down load a .pdf version of the factory service manual. The Prime Wiki may also have some still active links. Since you now own a vintage car, a factory service manual is an essential part of ownership.
