The DCP full container is 2048 x 1080. There are two main DCP aspect ratios: Flat (1.85:1 / 1998x1080) and Scope (2.39:1 / 2048x858). The DCP spec says that you have to have one side match your
If you are finishing 1080p, your height already matches, so there's no need to adjust the size. However, I've heard some stories about a very small amount of servers exhibiting strange behavior for 1920x1080 content because that's really close to the Flat resolution at 1998x1080. If I were you, I'd just keep your 16x9 / 1.78:1 aspect ratio and live with the super slight pillar box.
Many theaters don't mask to a perfect 1.85 anyway for their Flat presets, and you'll have a hard time noticing it onscreen.
If you have a 4k DCP, you'll still be able to play on a large amount of 2k screens. You'll only get a 2k picture, but it'll usually play (varies by projector/server combo). The original question was about how many 4k screens exist in the US and Europe, and the answer is that it is growing, but my estimate is that it's only 10-15% of the market.
If you're finishing at 2k or 1080p, it's definitely not worth an up-rez. I suspect that you're asking if it is worth the extra expense and hassle of finishing your film in 4k. That's going to be up to you and your budget, but if I were you, I'd shoot 4k, finish 2k and put all the extra money you were going to use for a 4k finish into your sound design / mix.
No matter how you get your DCP made (DIY or a professional service), you'll still run into the problem of making multiple copies (or pay the service a fee for every copy). DCP Transfer takes the hassle out of properly formatting the DCP delivery drive for Linux according to the proper ISDCF specs, as well as doing the final hash-checks to make sure that everything is all good before you deliver. Check out the overview video: https://youtu.be/NftjF4jFZVc
If you've used DCP-o-matic to create the DCP. You still have to deliver it on a properly formatted drive... That's where DCP Transfer comes in.
The newer versions of easyDCP include different conversions supporting the new standard 2.4 gamma. That's a pretty big feature if you're concerned about nailing color in the conversion process.
Congratulations David and Jason! If you need to make copies of your DCP to send out to all those festivals that want to screen Bill Nye: Science Guy, Cinematiq just released an app to do just that.
Check it out at www.dcp-transfer.com