The 4-digit grid conversion you're looking for is GRIDXXXX. It's available here at SUBSIM in two forms, the original by Tycho and modified by Fahnenbohn. Either "Search" or go to the "Downloads" section.
I installed it in LSH3 menu_1024_768 as G31 I35. But, I'd advise you to change the position before you install it; the original (-157, 1671) leaves it hidden beneath other pullouts at the upper left corner of the Nav chart screen. I changed it to 125, 1650, which centers it at the top of the chart.
Hope this helps; sink 'em all!
