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I'm sorry, but you cannot use DDR4 RAM in a DDR3 system. The two types of RAM are completely incompatible with one another. And the DDR4 modules will not fit any DDR3 memory slots at all (they are both physically and electrically incompatible with one another). What's more, your system is already maxed out on the supported amount and speed of RAM. Using incompatible RAM will result in a completely unusable system.
As for a SATA SSD, keep in mind that such an SSD in your old system will be severely limited by the system's SATA II interface: You'll never get more than about 270 MB/second out of a given SSD even if it's capable of over 500 MB/second in a newer system. That isn't sufficiently faster than a current-generation 7200 RPM hard drive to justify spending the money on.
You won't be able to use DDR4 RAM in your old motherboard, but you might be able to increase the RAM.
I that you can fit an SSD, but I am not sure how much of an advantage it would give in an aging iMac. I'm thinking not much. Rather than throw money at your old system, you might look at putting those dollars towards a new one. Does it absolutely have to be a Mac. You'll probably get more bang for your bucks with a Windows system, and Premiere Pro is heavy on resources.
I'm just visiting looking for information for my own new build, so tell the lads exactly what model your iMac is, and see what they have to say.
Macs in my opinion are just well-built systems with lots of power. I'm looking about getting one used. It is always a good idea to boot camp with Windows