Why can't the base screen just be put to a higher resolution?
I don't see how the battlescape needs changing unless you mean to prevent the tanks from getting stuck. I don't see a problem with that...player either learns not to get tanks stuck, or learns to free them once they are stuck.
Most of my answer could be Dioxine post. One point about "battlescape needs changing", I except that to do it right it would require rewrite half or battlescape code to made 4x2 units works. AI, Path Finding, Map and Unit rendering. And is big possibility that it will end up as big mess (3x3 would be more possible).
And finally one things that prevent it even if I could made it. This will break backward compatibility with basic version. its still should be possible to load extended save in normal exe, of course without any new functionality form extended version.