My suggestion:
El Capitan will run on your MacBook, but if you want your overall user experience to feel like "running" instead of "walking", you should replace the existing hard drive with an SSD.
Bumping up the RAM from 4gb to 8gb will also help, but not nearly as much as an SSD.
I'd suggest a Sandisk Plus or Crucial SSD, about 240gb in size.
Visit ifixit.com and search to find a drive replacement guide for your particular MacBook.
Mine was a mid-2010 MacBook Pro 13", and the replacement was "screwdriver-easy".
One more suggestion:
If you get an SSD, also get a 2.5" USB3 external enclosure.
"Prep and test" the new SSD BEFORE you install it into the MacBook.
You can "work out any bugs" this way.
When the drive swap is done, use the enclosure for your old drive.
It could serve as a "bootable external backup" ...