agreeing with the above, I'd also like to add.
I think it's because of the OS
back in the day, the OS isn't the one taking the most RAM resources so it's left for the apps to use. but now, the os uses a lot of RAM and the apps have to work with that's left.
4GB isn't quite enough even for the OS. as for windows 10, it would consume about 60%+ of that 4GB and you're left with less than 2GB RAM to spread across other tasks. also more of that space is reserved and it doesn't allow filling up the entire RAM, it writes to the page file. this further reduces the space hou have to work with to about 1-1.5GB (best case scenario).
my second guess is, the browser has many separate tasks running, beside from the webpages (which are not properly optimised), there's also many settings, and stuff the browser has to run to make the experience better. unlike back in the day, the experience wasn't that great and fluidness/smoothness of the browser wasn't the main objective.