Where is TRE4 set in mkgmap?

I know that TRE values 1-3 are set here:
uk/me/parabola/imgfmt/app/trergn/TREHeader.java

writer.put3(displayPriority);
writer.putInt(0x110301);

However there is a fourth value which in most Garmin maps is 0xd, whereas we use 0xc.
If I use gmaptool to adjust it to any uneven number (like the Garmin default 0xd) then routing does not work at all anymore. But Garmin maps use this value. Therefore I would like to try out what happens if we use mkgmap to directly set the fourth TRE value to different numbers, but I can't find it in the sourcecode.