I’ve got 4 GB of RAM

Sadly that’s an almost unusable amount these days. The OS will use half of that while sitting idle, and Google Play Services will use a fifth of the remainder (you can enable Developer Mode and view running services to see this yourself). 4 gigs is solidly in the “only one or two apps open at a time” range.

(It doesn’t help that every Android app includes its own separate copies of all its libraries and dependencies for compatibility reasons. There’s no such thing as a lightweight app without going out of your way to avoid the normal development ecosystem)

Android also doesn’t use swap (storing unused/rarely accessed memory contents on the disk for later), instead shifting stale memory pages to a compressed block in RAM (zram), so once it starts filling up with cached background apps it has to start killing apps or the system will hang. The less RAM you have, the sooner this will happen, and both Lemmy clients and web browsers use a ton of RAM.