It's easy to forget that it's not simply 3-car to 4-car, it's 3x23m to 4x20m. Of course, an extra car without a cab at one end means the 'usable' space is increased by slightly more than that (incidentally, a figure that tallies very well with the 16% increase in seating), but not as much as a 3-car to 4-car increase on a like-for-like unit.
The more I think about it, the units that seem better suited to TPE would have been AC equivalents of the 444s - the end-to-end distance of the routes is considerable, and the average inter-station length is also higher than that on the majority of routes where the 'standard' Desiro EMUs are used. I imagine the use of non-express style units is essential for dealing with the levels of overcrowding TPE suffer, as 444s would be a nightmare for dealing with the crush you see on 185s, but if delivery of the units in the first place is to solve the overcrowding issue (albeit after considerable time when everything is wired), one wonders why they weren't a little more ambitious.