Hi, Tigers! Glad you like the site.
In terms of "objective" value - assuming there is such a thing - I'd rank the three starters you mention a bit higher than Benitez, although it's close enough that I'd trade one of them for Armando if I were truly hurting for saves. One thing you could try is adding one of your lesser sp's, Appier for instance, to the offer, and asking the other guy to add a second-tier closer in return.
On the whole though, I see nothing wrong with carrying four closers as long as you have daily lineup changes. You'll dominate saves, get great peripheral stats on a per inning basis, and be in the driver's seat when mid-season trades come around. Sure, adding another top sp couldn't hurt, but your rotation is excellent as it is.
If you can make a deal of this nature, great - if not, you should still be in great shape, especially if your hitters are equally good.
Personally, by the way, I'd hang on to Benitez even if you could land a top starter, but take that with a grain of salt - I'm a sucker for closers and the tactics they allow you to use...