Those casters don't look 4". I have purchased many cases from Audiopile and have had no issues with the cases but I have had issues with the casters. Many of the casters from Audiopile have had bumps on the wheel surface. Some of the worst ones, I asked for and received a replacement from Audiopile. Others I have just purchased from SES Casters. I suspect you may end up wanting to replace the casters on that ProX case at some point.
That price would tempt me too. If it's at all close, I'd always recommend Audiopile. Mark & Liz are great people to work with.
Edit: The Audiopile cases have shipped with different casters over the years. I haven't purchased any recently so they may no longer have the rough wheel surface issue.