Thunderbolt solution for 4th Gen iMac

Sorry Joel, your options are limited to what the system offers: USB2 & FireWire800

The adapters Apple & others offer are always the reverse, allowing people with older drives & devices access to the newer systems ports. So as an example a FireWire drive dongle to allow access of the files on the HD to the newer USB-C interface on the new MacBook Pro's. In all cases the devices interface data rate can't be any faster.

So you could get a USB3 drive which offers a USB-A (USB2 & USB3) & USB-C cable so you can use it now with your system (USB2 data rate) with the idea you can use it with a newer system but the data rate won't be any faster than USB3 connected to a USB-C system.

Your other option is to go with a NAS drive which connects to your Ethernet port. Many NAS's are slow as the CPU's they use are not very good (cheap) the more expensive RAID'd and massive drive support units have much better CPU's to process the data stream.

Sadly, Firewire has come to its end. Apple pulled the plug on further development a few years ago. Even Thunderbolt & Thunderbolt2 are dying technologies.