TBH there are more reasons for me to stop using MPC. Everybody says keep it simple, and MPC only makes things complicated.
Well gotta say something I love about this board is the differing (informed) opinions.
So...I couldn't disagree more with what you said ;].
For me, the MPC has greatly simplified my workflow. I love being able to turn it on just anywhere, plug in headphones, hit a pad, and immediately get some sound (albeit after a 2 minute load wait).
So yea, if you want to simplify, do exactly what you are talking about. Sell the 4k, and you will have no problem adapting to the XL. It does the most basic stuff very well and you will already know it, I imagine.
For what you are talking about, you are doing the right thing. Sounds like you know what you are talking about, and here I am just agreeing with you...so I guess I don't know what else to say.
Be prepared for only a lowpass filter, and much more basic sound editing. Sounds like you do that in computer though, so shouldn't be a big deal.
As far as sequencing goes I think they are very similar in functionality, so that should be a non-issue.
If size is really an issue, consider the 1k too because it does most all the XL does, plus a little bit more. However, it does it all a little differently, so you'd have to re-learn things. Oh, and factor in an extra $100 + elbow grease to replace the pads, because the stock pads are really a **** joke...the fact that they even released them that way is, in my eyes, the sign of the death of the true mpc line. How can you expect to play an instrument that reboots whenever you hit it?!