DPI is a confusing issue - as it depends on the size of a picture, not it's resolution.
A picture 1440 dots wide at 20" wide will be 72dpi - but the same picture at 10" will be 144dpi
So what you need to do is cut the picture into 4 equal sections - then zoom them all to fit an A4 sheet, then print them. The resolution doesn't come into it.
I use MS PhotoDraw to do this. You could even use Word.