I need more zoom levels.

The maximum free available zoom level is 15. If you need more zoom levels, you can order Premium Maps. Premium Maps have additionally zoom level 16 and 17.

How can I improve 4UMaps?

Join the OpenStreetmap project. Use JOSM as map editor, download the area you want to improve. Add Bing maps as background layer and edit the map. Upload the new data and be a little patient (see below).

I improved OpenStreetMap data, but cannot see the changes in 4UMaps. How often are the maps updated?

The render cycle for the entire 4UMaps area is about 1-2 weeks. The data are synchronized every 1-2 days with the OpenStreetMaps server.

Can I get the render rules of 4UMaps?

No, 4UMaps are raster maps.

Since a while I cannot download maps with MOBAC any more.

Start MOBAC, go to "Settings", "Map sources config" and press the "Perform online update" button. This will load the latest 4UMaps settings.

My Internet Explorer 10 shows damaged maps after a while panning and zooming around.

This is an Internet Explorer specific problem. Go to the "Internet Settings" -> "Advanced" and uncheck the "Disable script debugging (Internet Explorer)" Checkbox.