My ViewingDale software is designed to do this job where you put in images at different scales and it allows you to zoom into one and it automatically loads in more images to make up the world until you get a seamless world. We have a large community driven world here on the guild called the CWBP and here is a video with the app running that through it. The maps were made by members here and I can put them in and scale them to fit.

http://www.youtube.com/watch?v=qELcbKXdRm8

There are other apps which do a similar thing but I wrote mine because I did not think that any of the other similar solutions coped well with this problem esp when you need to edit these zoomable landscapes.

Really tho it comes down to just how much detail you want and need to go into. You could do this with a vector app but even tho vector lines scale matematically it makes it harder to draw them with detail at all levels and still expect the app to cope with a zoom of the whole world. You need something where it knows about how to pick the detail and area for the bit of the map you need to look at. 8K x 6K is big but its not really really huge. You wont be able to zoom in all that far before you will start to run out of pixels for more detail with a single image. I would definitely recommend having different pixel scaled layers for your map. Just work out the actual meters/feet per pixel. If your doing a world with only 2000 miles across then 8K means 1/4 mile per pixel. Small towns will be one pixel on your map !