Need help for a big banner. Thanks

Greetings friends! I need your help for a project, i have to create an outdoor banner 3x15ft and i have to use adobe photoshop or illustrator. In Photoshop that 3x15ft file would be too much almost 1.5gb,

Can i create this banner like 1/4 of its size? Should i use 300 dpi for something like this? Or 150 works OK? I want to make the file as small as i can, because of my old PC.

Thank you for your time!

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