You must know what it's like if you have one

Too small for me

bit old and slow by today's standards but because it sold well it has pretty good community support.
I'm using mine now running android PIE and systemless root.

Should've got the fold. Excellent build quality, and it will last you at least 2 years without a blank screen (or simply a screen that breaks in two) that will cost you 500 quid to replace out of warranty

Tell me something, why do you always ask us about a device AFTER buying it? everytime I see you asking about a device I open the thread and see 'just bought one' and I'm like:

Anyway, great phone, nice and compact, would be best to go with what 3mel has done, as I have not booted mine up in a long time , so not quite sure how well the phone can deal with new apps etc...

I would imagine there is a bit of heating up and slow downs with more intense usage.

In terms of Hardware, the Screen is still fantastic, the S4 to me set the standard for modern smartphones, that screen was well balanced, smooth and crisp, it surpassed what was necessary in terms of Resolution, most phones have not moved that much further from it, these days it's more about brightness and content viewing standards and compliance e.g. HDR10, DCI-P3 etc...

Camera was great, on the S4 and battery life was pretty decent, there was also extended battery packs for it so that may be worth looking into if you find the battery life a little lacking.

there's a problem with eBay bought s4's, lots of repair places make FRANKENPHONES using the shell from one model, that motherboard from another and flash firmware a third to get one working phone (partially) in a specific region.

it's happened to about 5 or 6 xda users who couldn't flash custom roms because it had the wrong identifiers and hardware for the model it claimed to be.

google how to enter download mode and write down the model that that puts onscreen.
if it says i9505 that's great or i9500 is OK too just less versatile. anything else might not use UK frequencies.

there's also a issue an issue over whether to keep the phone on 4.4 or lower and have roms that can work with all the hardware perfectly (one lineage 4.4 rom is still being patched with security updates), or go 5.0 and higher which means you can never go back lower again and only up until 5.1.1 will be available as intended. 5.1.1 is the last set of official drivers for the hardware and roms above that lose functionality here and there like MHL or IR, face recognition. things like Wi-Fi or Bluetooth can be less stable etc.
chances are you're on 5.1.1 already anyway, if it's a genuine model.

you probably shouldn't buy batteries or battery packs for phones that are 5 years old now. anything that is around now will have been sitting on a warehouse shelf for years deteriorating slowly.

ive noticed a lot of independent shops are selling what they claim to be brand new S4s in the UK but where an earth are they getting the stock from after all this time? iam wondering if they are infact Chinese clones? they sell for as much as 100 english pounds sterling but you could easily pick up a decent used S6 for that price in the UK so iam thinking why would you go there in the first place? unless its just about nostalgia of course.

old flagship phones are still better than a lot of costlier mid tier phones. maybe it's refurbished stock or imports from another country that never sold.

I'd make the retailer sign something or agree to be videoed stating they are genuine before I spent any money.

I agree thats a good idea