Hi Emma,
Unfortunately, you need a bit more information to pinpoint where he'll be.
The main thing you need (strange as it might seem) is the cost/price paid for the lair. The most expensive lairs were around the outsides/perimeter areas and the closer they got to the centre, the cheaper they became. (If you look at an aerial or bird's eye view, you can see how the lairs are laid out)
A row letter (A, B, C, etc.,) would be useful but not always significant.
How sure are you of the lair number? I think there might be a missing digit as most of the lairs in the Western Section seem to have 5 digits. The only ones with 4 digits appear to start at 9000 +.
The Mitchell Library have the Southern Necropolis burials on Microfilm and are reasonably easy to search. You obviously have a date of death for your ancestor and as the registers are in date order of burials, it wouldn't be too difficult to get the info. The pages can sometimes be very difficult to read and in the worst case scenario - almost completely blank due to the film being so faded.
If all else fails, you might need to ask the Archives to carry out a search. I believe they charge £10 per search, but this
might be reduced as you can give them a name, an exact date of death and an note of the section in which to begin their search.
Glasgow University Archives hold Wylie & Lochhead's Burial Records.
http://www.gla.ac.uk/services/archives/Why not drop them an e-mail and ask them to have a look? Alternatively, you could visit their premises (just at the foot of Byres Road, across from the Western Infirmary/Art Gallery/Kevin Hall). They have a digitized index which makes for a much easier search. Once (and if) the burial is found, out come the musty old registers which you can pour over and they allow photos, so not only can you see the info, but have an image for your records (and with a bit of luck, if it's there, lots of additional info too!).
Why not post your 4x Gt Grandfather's details to this thread - you never know - someone somewhere might have additional information or advice of what else to try.
Anne