If you watch a 4: 3 signal stretched on a screen (or a projector) 16: 9, then technically you change the original content without permission of the copyright holder.
Instructions for the projector Sony VPLCW125 (took the first available)
“Please note that if a projector is used for profit or public viewing, changing the original image by switching to widescreen mode can be considered a violation of the rights of authors or producers who are protected by law" (" If you’re looking at