I have an odd assortment of attachments, most of which I'll be keeping (though twice the horsepower, the 4520 is almost identical in size (shorter though) than my Yanmar 336D.
As my closest dealer who was very helpful and great prices was a Woods dealer, over the years I've gotten mostly Woods implements, medium duty most, but not all, the time:
66" Boxblade
96" Landscape rake, with wheels
84" offsetting rear blade (if I have a 3rd circuit to the rear I'll finally put my spare cylinder on it- it fits already!)
60" Woods Brushbull brush hog mower. This is the one implement I might trade in if they'll take mine and I can get a larger one (6 or 7')
I also want to get a land plane.
I may get a Woods, only because they have their own version of iMatch which looks similar, and I'm guessing will work well with all their implements....
I'm actually looking to reduce my loader implements, from 3 to 2 - I only need a bucket and a grapple rake, not two buckets. I don't recall the last time I used the large bucket that has no teeth on it. I think the 4 in 1 will be great, and I may actually leave it on more than the grapple, but being able to swap implements out quickly vs. removing 4 pins will be a welcome improvement.
You'll definitely want to replace the two I've bolded. Wheel base is 72" and too have a mower not reaching outside of the wheels is really difficult - as well as a box blade to get close up to things. I'd definitely get new of them.
As far as the Woods vs iMatch they will do the same thing. I don't believe the Woods has the PTO set up as well, but I guess we can do a little bit of work. Either way I think you will have to make some adjustments to your equipment for either system to run the quick hitches. You only have to do it once for either though and your good. If your going to be using the mower a lot I'd see it as a plus to get the iMatch with the PTO quick hitch but either will do.
As for the box blade, Woods actually makes that box blade for Deere (although it is made iMatch compatible). So if you get the comparable through Woods it will be the same thing, different color.
Will you be getting the SSQA or the standard Deere QA? You will find a lot more stuff to go on the SSQA (4 in 1 included - and Deere branded) but they will come on and off in the same time. I do believe the SSQA is a couple extra bucks as well but could prove worth it to find more attachments down the road. Just something else to consider.