4GBs of RAM will be enough to run PhotoShop, but it will limit multi-tasking. When surfing, you'll slow down the laptop when you switch between 6 - 8 open browser tabs, especially in Chrome. Microsoft Edge actually uses less RAM than Chrome and Firefox, but it's still a 'work in progress'. If you use one major application at a time, e.g. PhotoShop, you'll be alright. But you'll have to shut it down if you need to run Microsoft Word.
About your most recent eBay find: With 8GBs of RAM, this HP seems to solve all your issues, but this is also an online auction. Even if the final price stays under £300, I have to say I'm a bit wary of this particular seller. This eBay account has 6 feedback comments as a seller, one of which is negative - very negative: Apparently, this seller sold a counterfeit bag, and my reaction is either the seller knows the item was fake (which makes the seller a fraud), or the seller didn't know (which makes the seller a clueless amateur). Either way, I would advise against buying from this seller.
Cheers,
GreyCatz.