The 4th axis could be handy down the road if you get into it, but it's specific to certain applications. You could take advantage of it if you plan to make multi-sided workpieces, although it may over-complicate things for small quantities. The other big use is batches of multiple small parts since you can pack them into the machine all at once, but with some work you might be able to do that with a typical 3-axis setup as well.
Having said that, the extra drive will probably increase the value a little bit. The HRT210 is an 8-inch rotary platter, so definitely not the smallest piece of hardware. Might be valuable to someone
Speaking about the gearbox, it'll make a noticeable difference for any high-torque tool like a heavy facing cut or a large-diameter drill or tap. It'll make no practical difference for anything running above 2500-RPM, so finishing operations and smaller tools won't care. Generally I tell people it's more effective during the roughing cuts rather than the finishing cuts.
I will say one thing, though, that is drawing a line in the sand for me in particular. Around this timeframe (1999) Haas was making the switch from brushed-to-brushless servomotors, so there are some machines that have INCORRECT "brushless 710-IPM" stickers on the front of the enclosure. They're actually brushed and rapid at like 400-IPM or something. The problem isn't the speed, but rather the older controller and motion hardware which is less reliable and no longer serviced. I have a machine where this was a major source of confusion when we bought a rotary axis specifically for it, only to realize it was incompatible...we ended up upgrading the entire machine to run the newer-style rotary ($$).
So I would be cautious buying a mill from this era because it may actually have older hardware installed despite the [literal] sticker on the front saying otherwise. I don't know how prevalent it is, but I'd prefer a machine from like 2000 or newer, since the controller saw some upgrades around that time. But yeah I know that means passing up on a deal...
I have two mills from around this era (97 and 99). They both still work decent for what they are, although I don't follow the used market enough to know what they're worth.