Digitize pictures

Hello,
Is it possible to to have a photograph scanned to such a high quality that the scanned file would make a good high quality vinyl banner of say about 6' x 4'?

Indeed.

You better scan your pic with the highest resolution your scanner allows you, but something like 1200dpi is enough - which will already result in a hefty large image. Then you use the adequate printer and you have it.

But have in mind that any defect the photo has will be greatly amplified with such a scan. Make sure your photo is free of dust and scratches before proceeding.