jpcanton,
The K4 38025 TMCC has a length of 23 1/2" vs. the Legacy K4s that have a length of 21". The Legacy K4s are scale the difference in length was an enlarged boiler between the front of the cab to the steam dome. I also thing the tender may also be a little larger than scale which resulted in the requirement for 054 curves. The Legacy K4s require 036 curves.
Unfortunately the Legacy K4s are now hard to find the last time I looked Nicholas Smith showed a pre-war K4 in their stock but that was a while ago. You maybe able to find a Polar RR K4 or a Tuscan K4.
Some of the opinions above are based on the Legacy K4s so do not make a decision based on this if you have or will be buying a 38025 TMCC K4.
JohnB