activity-monitorcatalinamemorysafari
I'm on 10.15.2 Catalina, and my fans have currently been running steady for hours. I opened Activity Monitor, am viewing the CPU tab, and sorting by % CPU. Safari is at 102.6 as I type this.
Activity Monitor used to break down Safari by the open tabs, so you could easily identify the worst offender. Unfortunately in Catalina I just see "Safari".
How else can I determine which tab(s) is/are the main culprit?
You can use Safari preferences (⌘ ,) to change this setting, you can use automatically or always for opening pages in new tabs instead of new windows.
I suspect you probably are using Mac OS 10.7 which is very memory hungry. As noted by kccqzy, the purge command may reclaim some inactive memory if you have XCode installed.
Something you should try is closing and reopening applications which are memory hungry or have a tendency to leak memory (Safari, Firefox). Alternatively, restarting your computer would also accomplish the same though take considerably longer.
To answer the question, are you really using 4GB of memory. Yes. Technically more if you take note of the "Swap used:" stat.
Best Answer
From comments…
In Activity Monitor, if you set the View to "All Processes, Hierarchically" then all sub-processes of Safari [or anything] will be contained within the main process, aiding the task of discovery.