These are great hubs. As others have hinted, they have a direct-drive low gear with 3 over-drive gears.... if my memory serves me, it's something like 1.0/1.25/1.5/1.85. Means you need to go smaller up front and/or smaller out back, but that's probably how come they're so reliable. It's often the high-torque situation of underdrives that fry gear hubs.
Also, like others said, these hubs have been DC'd so you can't find parts. If you only lack a shifter, you MIGHT find one, but they're pretty rare these days. Basically, if/when you need a part for a Nexus 4, you're SOL. (Take notice: Shimano has always been cheesy in terms of providing spares for their IGHs, even on current production stuff. But, once they stop making a given IGH, you can forget buying new stuff for it. All you Nexus 3, 7, & 8 ppl might want to pick up some spares now, before they're all gone.) If you can find a shifter for it, I'd say ride it til it dies, then get whatever hub you really wanted. I like Sturmeys, myself; not necessarily more reliable, but easier to service, you can buy just about every/any part for it, you have a bunch of braking options, and the 3 speed shifters from 75 years ago work on all their new 3 speeds, except that s3x fixed gear thing. So, more parts support, and less planned obsolescence.
