What are the smallest dimensions/resolution I can use for a 7ft x 2ft banner?

I am designing a banner which will be printed at 7ft x 4ft.

I am using Photoshop CS4. The images are not vectors. The original poster artwork has been done at 300 ppi for A4 printing. I was hoping to re-size the canvas of the poster (after saving as a new file) to keep the 300 ppi resolution but working on a smaller scale to reduce the file size. This is mainly to avoid having to copy all the layers in to a new file to recreate the existing image.

I have looked around at various questions already on the site, but can't seem to find an answer. The common recommendation seems to be that banners or other large-scale pieces that will be viewed from a distance only need to be around 75-100 ppi, but I was hoping to work with a higher resolution and smaller size, instead of lower resolution and larger canvas.

Any and all advice or suggestions are greatly appreciated.