I personally wouldn’t use a phone with 4GB unless my usecase was fairly basic. If you are running multiple (Heavy) websites and apps the system will need to use swap will slow down multitasking.
A couple of months ago I revived an old x86 tablet using postmarketOS. No Android anywhere. The only somewhat usable browser I could find was Falkon. Firefox was absolutely unusable. Ungoogled Chromium was a bit better but overall still shitty. Opening a second tab under Falkon was too much as well. The problematic component was 100% the web browser and there Firefox specifically. OP writes that he opened Firefox and I can confirm from my own experience of using Firefox on a RAM constrained non-Android device that FF is terrible in such scenarios. FF is great otherwise but not when RAM is low.
A couple of months ago I revived an old x86 tablet using postmarketOS. No Android anywhere. The only somewhat usable browser I could find was Falkon. Firefox was absolutely unusable. Ungoogled Chromium was a bit better but overall still shitty. Opening a second tab under Falkon was too much as well. The problematic component was 100% the web browser and there Firefox specifically. OP writes that he opened Firefox and I can confirm from my own experience of using Firefox on a RAM constrained non-Android device that FF is terrible in such scenarios. FF is great otherwise but not when RAM is low.