The 5th pod seems less popular than it used to be. Better worldwide availability of spares, better engine reliability, reduction of the 747 fleet, and the arrival of (relatively) cheap outsize Antonov 124 capacity on the charter market to haul large engines around, have all played their part.
In the early days of 747s such were the problems with the JT9D that sometimes, when ferrying out a spare to hold at an overseas base, the engineers would suddenly end up taking it off and fitting it to the same aircraft, which had suffered an en-route failure. And then ordering another spare.
In truth, although I have tended to look up at any overflying aircraft for more years than I like to admit, I can only remember seeimng the 5th pod a couple of times. It's one of those things, like operating with a winglet missing, that can surprise those who see it for the first time