Thanks to halv12 for the link to the "Market Fails" thread, this identified that Market buffs only apply to workyards with their bottom corner within the buff area. Therefore, the L4 11x11 grid, being only 1x square ALL ROUND bigger than the L2/L3 9x9 grids, forces you to waste 3x potential building slots - the attached screenshot will hopefully explain what I mean.
Here you will see my L4 Christmas Market surrounded by a regular(ish) grid of workyards. The outer red line indicates the workyards that are within the Market's 11x11 buff grid. Those on the bottom L+R sides are fully within the buff grid, whilst those on the top L+R sides are only within for half their building footprint, but including their all important "bottom corners".
The purple lines indicate a 1x square empty space all around the Market - ie the Market is positioned in the middle 2x2 squares of a 4x4 grid thereby wasting three potential 2x2 building slots if you were to place the market in the corner of the 4x4 grid. However, if you move the Market one square in any direction the 11x11 grid then fails to cover the bottom corner of all the workyards it moves away from.
I don't think I am missing anything obvious here, but it seems to me that it is simply not possible to cover more workyards that "two rings" around the Market because of this. The theoretical maximum number of 2x2 workyards an L4 Market could possibly buff is therefore 32, but I dont think there is anywhere on the island that can accommodate that. Here I've managed to get 26 workyards and I think that maybe the max possible, even with cleared mountains in zone 9.