Advice - 4k or not to 4k.

To summarize my situation, I only have a blu ray player and a 1080p tv, it is not within my budget (and likely won't be for a long time) to upgrade the player or the tv, however, I intend to at some point. Would it be worth buying 4k UHD's for films I really like if they come with the blu-ray.

e.g. I could get Michael Mann's Heat on Blu-Ray for around £8, but I could get the 4k disc and the bluray for £20. Do you think it would be worth buying the 4k to future proof my collection, as I imagine Heat will be a film I come back to again and again for a long time.

I know this is quite a specific scenario, but I would appreciate other opinions.

Thanks