Q: upgraded my iMAC 7,1 with 4 gb memory with the intention of going to El Capitan, but after reading how it slows down the speed, was wondering if going to Yosemite is a better choice.

If you upgrade the amount of RAM your iMac should be able to handle El Capitan nicely.

You probably only have a few days to upgrade to El Capitan. After that, most likely it will no longer be available for you to download.

Read Upgrade to OS X El Capitan - Apple Support.

Assuming that you are already in the habit of routinely backing up your system you can evaluate El Capitan at no risk and at no cost. If you do not routinely back up your system, you should. Obtain an external USB, FireWire, or Thunderbolt hard disk drive for your Mac, then download and install El Capitan on it. You can choose to start your Mac from that disk or the internal one using Startup Manager (hold an option key while you start your Mac), and evaluate El Capitan at no risk whatsoever to your existing installation. Just be sure to designate the proper installation location for El Capitan when you get to that installation step.

Running OS X from an external USB hard disk won't be as fast as the Mac's internal one, but it will be sufficient to draw conclusions regarding its suitability for your needs.

An external hard disk drive of adequate capacity can be purchased for well under $100 — much less than the cost of some previous OS X upgrades themselves. When you satisfy yourself that El Capitan works for you, then "clone" the external disk's contents to your internal one, and use the external disk for backup purposes — which you ought to be doing anyway.

In addition, you iMac can accommodate up to 6GB . OWC makes a kit or you can buy a 4GB RAM chip from them and install it.

Considering the computer's age though I don't see a lot of value in putting more money into it. I would begin considering replacing it with a more up-to-date Mac that can accommodate up to 64GB of RAM.