Use the same gamma idea on an ideal rod measured along its motion. A 5 m rest
length divided by gamma 5/4 gives a 4 m moving length, while real length checks
depend on simultaneity in the lab frame, calibration, alignment, resolution, and
uncertainty.
A five-meter rest rod measures four meters along the motion. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.