I was told by a VZW employee that the reason 4G is spotty in South Portland is that lease issues are/were preventing them from upgrading the two towers in South Portland. This was a few months ago as I have not been to Maine since August.
The reason the VZW has coverage in the Maine Mall is the VZW store has a microcell in the store that has its own back haul.
Biddeford has excellent 4G coverage. When I first moved to the Northeast, Biddeford used to be extended network on Verizon until they filled the gap in York county.