If I were to run a standard 6-GPU miner, what would be the expected data usage, bandwidth and latency requirements? Is this something that could feasibly by run of a cheap 4G plan?
Assuming you refer to pool mining...
This question was partially answered here: Does mining use a lot of bandwidth?
20 Mb/day download and less than 5 Mb/day upload
So over a month, that's ~750 MB combined. So a 1 GB 4G plan would be more than adequate.
As for latency, the general rule is that you want the minimum latency possible. Any latency in your connection results in losses in mining efficiency. That being said, 4G connections quote latencies in the low 100s of milliseconds, which is probably good enough. This assumes a strong 4G signal, at the least. It's possible you would be okay with a 3G plan as well, as that tends to only ~double the latency. The size of information being transmitted between your miner and the pool at any given time is quite small (much less than even a fairly simple webpage) and so many of the benefits of 4G vs 3G are likely to not be as noticeable.