The Mk4 was the first move to slightly bigger production runs, each batch was slightly different but still reflected the original Mk1 design concept. However to aid production, build costs and for continuous production improvement several technical changes were made.

The tie bars for the LHS exit were moved across to a single tie bar at the left hand end of the box.
Like the Mk3 the silencer material was made from seam-welded rolled sheet rather than heavier and much more expensive 6" stainless pipe as used in the Mk1 and 2. This helped reduce weight and production cost and effort.

All Mk4s came mirror polished after the very positive reception for the Mk3 "Exhibition" and "Polished to Oblivion" (#19 and #22) examples. 
As with all TT exhausts all Mk4s are made of 304 mandril bent stainless steel but all were and presented mirror polished when new.

Their straight-through 57mm diameter format guarantees a loud beefy traditional MG sports car sound.
After some prototyping Mk4 finishers were designed and offered. These could be retro fitted to Mk1 and Mk3. Also for Mk2s without welded on finishers.