One useful trick for cases like this is to record something on all the tracks and then flip the tape upside-down. When you play it back, track 4 on the head will be playing back material recorded by track 5, and vice-versa.
If track 4 plays back under those conditions, and track 5 is silent, we know that the head itself is good, that the repro amp is working and it's something iffy with the recording amp on channel 4.
Conversely, if Track 5 plays back material previously recorded by track 4, but track 4 is silent, we know that the record amp on channel 4 is working but the repro amp is not.
If both tracks 4 and 5 are silent, that means that channel 4 can neither record nor play back. Which likely means that the connection to the head has failed somewhere (or the head itself is open-circuit but hopefully that's not the case). In that situation it would be a good idea to swap the channel cards around and see if the fault moves with the channel card. Assuming they are not soldered in on the R8, of course. If they are socketed it might even be the case that they just need re-seating.