Been racking my brain on this

You will note that Mac OS is not listed under the "recommended" minimum requirements:

http://fortnitehelp.epicgames.com/customer/en/portal/articles/2840925-what-are-f ortnite-s-minimum-and-recommended-specif…

I'd say that, aside from the fact that Macs are not known to be the best "gaming" machines (stick with a Windows machine), increasing the RAM might help. An SSD (not a fusion drive) might also help as would a faster processor (i7).

So like everyone else in the world, I've been playing and loving the game fortnite

Not everyone.

With gaming, at least 16.

Is this a 21 or 27 inch screen iMac?

Try adding in more RAM

You MUST use THE EXACT RAM that your model iMac is spec'd for.

Proper spec’d and reliable Mac RAM can ONLY be purchased from online Mac RAM sources Crucial memory (crucial .com) or OWC, aka, Other World Computing (macsales.com).

When buying RAM for Macs from Crucial memory, purchase ONLY directly from the Crucial memory website. NOT from some other online source.

RAM modules are, relatively, easy to install.

Good Luck!

Upgrade the RAM, 8GB is a bare minimum with current version of Mac OS. If you decide to do this please only use www.crucial.com or www.macsales.com RAM. The reason for recommending these 2 vendors is iMacs are sensitive to using quality RAM and both vendors supply high quality products with reasonable prices and offer excellent service.

Especially when you are using a machine that is not optimal to what you want it to do. Be sure to check here:

https://eshop.macsales.com/shop/apple/memory/iMac?_ga=2.61144172.1314239660.1523 632845-439413117.1522008578

It is never a bad idea to find out if you have a software conflict before throwing money at hardware upgrades. Back in teh day I had some games stuttering and it was due to an old fax modem left attached to the computer but not the phone line. Every 20 seconds the computer polled the modem port, stalling whatever was runnning. One modern equivalent is torrent software that is active in the background.

Posting a snapshot of your system configuration is a good place to start. Fortunately there is a safe and secure way to do that.

A respected and long-serving member of these communities has created a simple utility that will take that "snapshot" of your configuration without revealing any sensitive information about you and your computer. It only runs when you tell it to, not in the background, and therefore creates no performance penalties. It is Etrecheck, and is available here:

http://etrecheck.com/

Run it and, when its results display, select "Report" from the left-hand pane (scroll down), then click Etrecheck's "Share Report" icon followed by "Copy Report" from the resulting dropdown. Paste the entire report into a response to your own thread here. It will often allow us to quickly identify or eliminate software as the problem.

I didn't even know what Fortnite was until about 2 weeks, or so, ago!