I have noticed some websites come up at www1 or www4.domain.com - what is this? Does it affect SEO at all?
It is just a sub domain, like www, You might be knowing this, It is like any other sub domain, so, It does not impact your SEO.
It most likely means that this website has multiple hosts (servers) and it just depends on demand as to which server responds at the time you request the page.
Most likely, your HTTP request is answered by a load balancer which route the traffic to the less-loaded server. Most of the time the load balancer enable session stickiness which means if you hit ww1, all your others request will hit the same host.
Different servers and/or load balancing. Either way, it's a little sloppy to have that shown to the user that way.