I only have an A4 scanner, too. Here is what I do to get a good result:
- Draw liitle crosses in your map.
- Scan
- Put the 2 (or more) scans in different layers and set the upper ones on multiply
- now you can move and transform them until all the crosses are exactly on the same spot
- use a soft eraser to erase the edges of higher layers and a hard eraser to erase the crosses.
This of course only works if you use PS or something similar.

Great outlines.