Well I got the crash issue fixed. I had set the center of the tile wrong. Also, the 4 mile square objects snap to the red grid in the Terrain Editor. Good to know.
I took an existing 4 mile object from the list, modified it a little, re-saved it, and placed it in a test terrain. So the process works.
I think all things considered, I am better off taking existing 1 mile x 1 mile, 2x2, and 4x4 objects that have already been made, and modifying them for my use. Example: take the 4 mile 'tland0' object, exporting it to AC3D, doing what I want, and then saving it as a new name. It seems to have less problems than starting from scratch in AC3D or L3DT Pro. I'm sure it's a naming convention I'm getting wrong, because in the text file for a 'from-scratch' 1 mile object, the materials and textures seem to have 'AC3D' in their names, while the modified objects all have references to "AH23", "AH42", etc. Is that the issue? I don't know enough to know.
Maybe one day I'll figure out how to create a land object from scratch that doesn't have problems, but for now, modifying existing land objects works just as well for me. And it's actually quicker.
Side Note:
Here's why reworking the 4 mile square objects is so key for me: it allows me to create a large, custom-molded block of terrain that I can place to the side of towns and fields. If I put the spawn point roughly 4 miles from the town/field (and since the TE requires at least a 1 mile space between the edge of the town or field, and the edge of my custom 4 mile block of terrain), it provides me with a very large area for quality GV terrain.
Also, since the tankland objects are already on the approved list (I think) for the main arena (although not used), I have hope that I can get a different tankland-"ish" object approved for use in the main arena. Which was my original goal. So a future without tabletop-flat, densely-treed terrain between the spawns and towns/fields is within reach.