I recently bought the upgrade kit for the Tec 4, and thought some people might be interested in hearing about my experience. I don't have a time for a long post now, but will provide more details later.
I bought my printer as a dual Felix 3 back in 2015. I upgraded to a 3.1 when that came out. My printer was working pretty well, so I didn't really have a need for the upgrade, but it had a couple of compelling features and I was curious. This is my hobby, so I was able to rationalize the expense.

The upgrade process was relatively painless. There were a few stumbling blocks along the way, which I will detail in a later post, but nothing serious.
The description they give of the auto leveling capability for the Tec 4 on the web site isn't super clear, so I thought I'd provide some extra detail. When you select the bed leveling calibration from the LCD, it warms up the bed and extruders and does a 3-point probe. It then gives you instructions on precisely how much to turn the two leveling knobs, which have markings in 10 degree increments. It then repeats the process until you get to within a certain threshold, after which it appears to do a bump map that I am guessing is stored in the EPROM. So it's kind of like what the Pro 2 does in terms of getting a mechanical level instead of a software correction, but without the actuators.
I've only done a couple prints so far, but the quality of what I've gotten is very nice.
The big thing, though, in my mind, is the magnetically attached removable build plate. This is simply great. The big flaw with the previous Felix 3 design in my mind has always been the susceptibility of the aluminum build plate to warping, and between the inductive probe and the removable plate I'm optimistic that I will be able to print with confidence over the entire build surface (which looks smaller than the 3, but it seems they've just trimmed away the area that wasn't printable anyway).
I've got a few nitpicky comments I'll save for the next post, but I'm now at the point where I think I could really recommend this printer to friends. Here are what I think are the key features:
(1) Removable, heated build plate
(2) Portability. The handle makes it by far the easiest printer I know of to move around, great for taking to schools or clubs or wherever it's needed. I don't know of any sizable printer that has anything like this.
(3) Reliable (I hope) dual extrusion.
(4) Solid frame and direct drive for high print speed with good accuracy
(5) Modifiability and Upgradability. The aluminum extrusion is easy to attach stuff to, and Felix continues to allow upgrades.
(6) Price. With the Tec 4, you're now getting a printer that I think compares well to an Ultimaker or Taz that you can build yourself with a substantially lower price tag. The upgrade felt pricey (it replaces all the expensive parts of the printer), but buying new seems like a real deal.
My printer is over two years old now, but it feels like a brand new machine. Happy times!