Yes all blu ray players will down scale to 1080 and your 1k tv will then downscale to whatever it's resolution is. I would avoid buying a 4k player right now as there is only one released and it is very expensive. Most of the 4k disks you can buy right now come with a double pack that has a normal bluray disk and a 4k disk in the box. So you are free to buy these and use these on a nice cheap bluray player. Once 4k players and TV's are in your price range in a year or two then it's simple to make the change then and enjoy your new disks.
Please note that for 90% of people with 4k setups they will not be able to see the 4k and it is just not worth worrying about as the resolution is too high for you to see it. You need to either have your chair very close to your tv (like 6-8' away) or have a giant > 80' screen. If the screen is not taking up a large portion of your field of view then 1080P or lower will be all your eyes can see.
Here are a couple of articles/calculators to help you find what size tv and viewing distance you need.
https://www.audioholics.com/hdtv-formats/how-to-find-the-right-size-tv
http://referencehometheater.com/2013/commentary/4k-calculator/
measure how far you sit away right now and the size of your current TV and see what resolution you need to not miss any detail.
The reason 4k is doing so well right now is because when they walk into a store and see how great it looks from 3' away people are willing to put down their money.
The only 2 reasons I can see to logically think of buying a 4k TV if you are not close enough is for HDR or if you wanted to watch 3D content. HDR in theory gives better colors and better dynamic range so you can have better dark and light scenes. I'm not sure how good HDR will really be though as cheaper LCD based panels don't really have the contrast ratio to do it right and OLED are very expensive still. Plus content for HDR may be very hit and miss and hard to find.
3D viewing used to be a big compromise as you either used active glasses technology where the screen flashed between each eye and had many ghosting and other issues. Or you had passive technology where the glasses are very cheap and the screen alternates lines between left and right eye. Passive is a lot easier to view but you only see half the 1080 lines making your resolution more like 540p which is SD. This made text and detail hard to see. These limitations are one of the reason 3D has almost died off now. But 4k passive 3D solves these issues and you can now view 1080p 3D content on a passive screen. However it is getting very hard to find screens supporting this now as the 4k/HDR has become the new replacement buzzword.
