I'm certain that you already looked into the overhead clearance needed with the 4500. I don't know the heigth of your doors or ceiling but the exhaust and roll bar stick up pretty high. Do the buildings that you're cleaning out have cement or dirt floors. I can well imagine that this manure sets up like concrete after a few months and 4wd would definitely be an asset to you. I think you could justify the extra cost with the 4wd 1,000 times over. As the FEL gets to capacity, the backend gets lighter but with the 4wd, the front tires only get more traction. The 4500 is a darn good model as I have one and I have never regretted it.
As far as the trailer capacity, the only thing the other guys and myself are recommending is because we learned the hard way. I would recommend that you don't even buy one if it is not an important accessory for your needs. To license it each year, you could borrow or rent a trailer for that once or twice you might need it. A 12,000 lb capacity trailer is what I'd recommend as it will easily carry your equipment - no worries about tire blowouts, etc and much more easy to resell.
I'm seeing more and more "package" deals on TBN lately and it's a nice way to purchase a bunch of equipment at one time. It might surprise you however just how much you are paying for each individual piece of equipment. You are buying what the dealer sells and it gives you no chance to buy something else that might be a better buy.
Best of luck with your purchase - Clyde