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Hi Aidan, I would see little problem with projecting using a 4 1/2 inch scope. I've regularly observed the Sun using the projection method with a 5 inch and 8 inch reflector for a great many years, and never broken or damaged any equipment as a result. I'd recommend not using a favourite, new or expensive eyepiece for the job, just in case the worst happens, as the greatest risk is with the concentrated light at the focus at the eyepiece end. I don't think there's any risk to your primary mirror or objective lens. You'll not set anything on fire unless it's very close to the eyepiece: by the time you've spread the light (by projection) out into a broad disk it's not hot enough to ignite the paper. For lurkers, it goes without saying to never ever look through directly of course - the damage threshold to a retina is a lot lower than a piece of paper!
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