People here are getting HDD and RAM confused and mixed up.
RAM = what's currently running. You have 4 GB RAM, which is the amount of memory currently-running programs can make use of. Chrome in particular is known to be memory-intensive because it creates one process per tab. The only way to reduce memory usage if you're using Chrome is closing tabs.
HDD = what's saved on your hard disk. This is everything on the system, including programs that are currently running (just the executables, not what's currently onscreen) and what's not running.
You can't free up RAM by clearing your Internet cache because that's stored on the hard disk.
I'd recommend upgrading to at least 8 GB RAM if possible. If your computer has two RAM slots and only one slot is used (with a 4 GB module), it shouldn't be too difficult to upgrade.