WestYorksRoyal Got me thinking, we must have played some of them in the 4th tier too. How long would that list be?
Four clubs by the looks of it: Portsmouth; Swansea; Watford; Wigan.
In addition, we played Portsmouth; Swansea and Watford in the Southern League before joining the league. Swansea and Watford are the only sides we've played in all four levels of the league; the FA Cup; the League Cup and the League Trophy (we haven't played Portsmouth in the Trophy).