I have a 2520 which is equivalent to your 4115. Both machines take the JD 46 backhoe. I've used mine for a couple small projects by now and I'll give you my insights.
I started out looking at the 3000 series tractors and the 48 backhoe. The 48 digs 8ft and you can attach the BH without leaving the tractor. You can also leave your MMM deck on while having the backhoe (although you probably shouldn't have both on at the same time).
At the last moment I switched to the new 2000 series and didn't notice that the backhoe conflicts with the mower deck. Also, the attaching of the backhoe is more involved that any other attachment:
- remove the 3pt arms & drawbar
- lift the backhoe onto the tractor using the stabilizers
- line up four connection points by jockeying the backhoe with stabilizers and/or bucket arm
The first time I connected the backhoe, I was reading the instructions (yeah, I know...) and it took about 1/2 hour. I can repeat it now in about 10-15 minutes. It's not the 3-minute routine you get with the 47/48 backhoes, but it's not bad. I use the backhoe a couple times a year and spend most of my tractor time just mowing.
The 46 backhoe has 6ft dig depth and about 7ft reach straight out. Once you substract 2ft for the half-width of the tractor and some more for the stabilizers, you'll find that you quickly fill up the reachable area with spoils from the hole you are digging. You won't be able to reach across the hole, so 90-degrees left/right of tractor is only place to drop dirt.
Moving the backhoe means jumping off backhoe, lifting stabilizers, fold-up backhoe seat, sit on tractor seat, brakes, gears, loader, re-position tractor, reverse all of the above. If you are digging ditches (my first project was a 20ft culvert), it can get a little tiresome.
The power of the 46 backhoe is decent. I've moved the tractor instead of the stump a couple of times! You can't fault the hydraulics when they pull the tractor with brakes on, outriggers down and FEL filled with debris. My hydraulics can be a little jerky, but that's part of the 'amateur' set-up. If you want fast, smooth professional hydraulics, you are going to pay $40,000+ for something that won't fit in your driveway!
Having said all of this, I'm still happy with the $6,000 backhoe. I've plucked fenceposts broke flush with the ground, removed 12" dia stumps (there is a 28" dia in the backyard still waiting for me), replaced a 18"x20ft culvert, and used the backhoe to dig/move/place (6) 2.5" oak saplings with 30" root balls.
Being able to put each root ball exactly center in each hole while keeping the tree upright is well worth it--I spot the tree and my 90# wife runs the controls. It's almost a one-man show, and we planted 1 tree/hr. Sure beats hand-digging!
I wouldn't want to deal with renting, delivery and racing the clock to have a mini-excavator for my misc. projects. I glanced at a couple used mini-excavators for sale. These were $12K and up which was way beyond the budget. I'm sure they would be nicer/faster, but I don't have the size/qty of projects to justify those big bucks.
The decision is yours. Let me know if there's anything else you would like to know. Be informed and good luck!