Originally Posted by
AeroWesty
Don't put that routing in. Use "UA UA UA UA" without the quote marks in the routing code boxes to indicate that you want 4 UA segments, while using TPA as your origin and SFO as your destination. Heck, even try 5 UAs, see what it comes up with. Like I said before, these routings are combining two fares. Let ITA do the heavy lifting for you and show you what's possible.
ETA: I just re-ran TPA-SFO asking for 4 UA segments, one-way on Feb 15. The highest mileage routing it returned was TPA-CLE-PHL-IAD-SFO for $280.20, giving you 4,146 miles with the 500-mile minimums. That's combining fares of TPA-PHL + PHL-SFO.
With very rare exceptions domestically, asking for more than 3 UA's will either yield a combination of 2 base fares or a direct flight which unless you split the segments, won't get the extra EQM.
My end of year 2012 mileage run was PHX-LAX-IAD-TPA-IAD-SFO-PHX. While I haven't checked, TPA-IAD-LAX-SFO may be a legal routing which I would expect to be the best you can do.