4k on older computers

I have a 2009 mac pro , yosemite , latest final cut & 32 gig of ram

at some point I plan to upgrade to a machine that can truly handle 4k - but there's some stock footage i want to buy today. I have the option to buy it in standard hd or 4k, but if i get it in SHD I'll need to re-purcahse it later at 4k for the newer machine.

is it safe to buy 4k footage - can my mac handle it, or at least convert it to soemthing it can handle?

never used 4k before.so i have no idea what it entails to use it, but i'm fairly certain my current machine would choke on it.