It really depends on your goals and your situation. If you have to fly in, that 4 day cruise is preceded and followed by long days travelling - for us, even a 2.5 hour flight means a full day of travel and/or a very early morning. So I have to take more time off work, I have to decide if it's worth the hassle and time of the travel, and whether it means that I'll be rushing out of work to get my flight and then heading back to work the very next day after the cruise. So, previously, we would never have considered doing this without at least a few extra days at WDW.
However, we've just recently bought WDW APs and have been going more frequently for 4-5 long weekend trips and spending a lot more time in the resort and only a couple of hours in the parks. That has actually been really relaxing and really enjoyable. It's still a pain for the flights and the fact that you get essentially 4 days for a 6 day trip but the time spent doing whatever we feel like has been good. I can see a 4-day cruise being similar if that's what the goal is. And we're considering doing this once both DH and I are retired. I may not care about a couple of days of travel if I'm not worrying about getting people prepared at work for my absence for a few days, checking my e-mail when I'm gone in case they need something, coming right back to work when I return.
While we prefer longer cruises, 7 day and shorter cruises will generally be our don't-do-anything cruises. No major plans. Given that they are typically Bahamian/Caribbean cruises, we probably wouldn't spend much time on the islands since we've either been there before or there's not much to do that we're interested in. So we'll spend it relaxing on the ship and doing whatever we feel like (or don't feel like). If I want to see places and do more things off-ship, I'll want a longer cruise.
So for us, it's not an either/or situation. It's going to depend on our goals (relax versus see/do lots of stuff/new things), where it's going, and how it fits into our general overall life schedule. We may do both.