I'd hazard a guess it might be due to Asian market considerations. In Chinese the number 4 is claimed to sound like the word for death. It is thought best to be avoided. Some buildings in China skip the 4th floor, just as some western buildings skip the 13th floor.
The Thiel 3.5 was the last Thiel to use a bass boosting equalizer, but certainly not the only model. I believe all the 3 series preceding and some of the earlier, smaller models did as well.