Ray,
Welcome to the forum - glad you are here!
To get a cover large enough to go around your book you will probably have to print at A2 size and then cut down the printed sheet.
I can't think of a way to print this using your existing A4 printer (unless the spine is going to be on the short side of the page - then you can cut the paper down to the right width and print the cover.
If that is not an option, try locating a digital copy center that can just print off the cover. Sometimes, a photo printing center can create a large format print from a digital file. Otherwise, try to locate somebody locally who has a large format printer and work out a deal to get a few copies printed.
Hope that gives a few ideas for getting your covers created.
Andrew
