I would check your power supply cable to make sure you have additional SATA power plugs before buying three more drives.
If you lack the plugs: then transfer your 500G to the new drive for 3.5/7.5GB net upgrade. You could probably add a Molex/SATA power Adapter but I think dropping the small drive would be preferable.
The biggest improvement might be switching in a 2TB SSD to hold your whole current system and then assessing your storage needs past that. A large SATA SSD (upgraded in stages as prices dropped) has kept my older system adequate for a very long time.
Something like the Crucial MX500 2TB is now only $150. (The 4Tb is only $270!)
MX500 2TB on Amazon