I'd argue that Top 4 is the most successful. Whilst i agree that you don't get any prizes for finishing 4th, and it won't go on our honours, for me, pitting our wits against 19 other teams over the course of the season, to Qualify for the Champions league is a great achievement.
I'd love a trophy, i have never seen us win one, but as we've seen so far, We've faces QPR, Stevenage and now Swansea is upcoming, in the kindest respects, it's not the same as turning up at Arsenal and United, and making them brick themselves, we could end up getting a League one side in the Quarters, and then a Championship in the Semi's/Final. Whilst winning the cup would be a momentous event, in these situations it's not the biggest success we can get.
I'll probably get lambasted for this. But to build on our success, and not be a flash in the pan like Swansea or Wigan etc, Top 4 could cement huge cash windfall, which could set the club up for years, propelling us to trophy success for years to come. With this squad it's easily possible.
For me: Success is a trophy.
Bigger picture: Finishing 4th.