I received my MK4s 10 days ago. I started with the 1.5, graduated to the 2.0, and now the 4.0. As much as I liked the Def2s, the 4s are a totally different speaker. The only similarities are in size (virtually identical to the 2.0) and looks.
The engineering is amazing. (1) The integrated amplifier can be unscrewed and popped out for service if necessary. (2) Similarly for the bass driver. (3) The aluminum base that supports the downward-firing bass driver is a work of art. (4) The Neutrik Speakon connector preserves the geometry of the cabling and accommodates a selection of connectors for each different type of speaker termination. (5) Bass can be tuned to the individual room. Etc. Everything has been thoroughly thought out.
The highs and lows are more extended. Deep organ notes are palpable (when appropriate). Resolution is better, but the sound is also smoother, more organic, and even a little more dynamic. The soundstage is deep and wide even though they are within 4" of the back wall. Off-axis imaging is amazing; maybe it is the way that Sean set them up on my 24' long wall, but the image does not change whether I am between the speakers, sitting in front of one of them, or standing against one of the side walls. And yet, everything that I liked about the Mk2s is still there. The sound easily fills my living room and the main living space of my house without a change in tone or quality when I am in a different room. I have listened for hours without any fatigue. Truly amazing.