What are the effective scaling modes on OSX with a 4K display? Is an effective real estate of 2560x1440 possible?

Note that if you Alt+click the "Scaled" resolution radio in the display preferences you will see many more resolution options than the default 5 (source: Apple support article "Using 4K displays, 5K displays, and Ultra HD TVs with your Mac").

The options depend on the maximum resolution of your monitor and possibly your Mac graphic card/Mac generation etc.

For my MacBook Pro (Retina, 15-inch, Mid 2015) and LG 27 inch 4K monitor I see the following options:

Full list:

• 3840 x 2160 (4K)
• 3360 x 1890
• 3200 x 1800
• 3008 x 1692
• 2560 x 1440
• 2304 x 1296
• 2048 x 1152
• 1920 x 1080 (Full HD)
• 1680 x 945
• 1600 x 900
• 1503 x 846
• 1280 x 720
• 1152 x 648

And to answer your bonus question:

I am guessing that you use 2560x1440 on a 27 inch display now.

If so then to be comfortable you should get also 27 inch 4K monitor and use scaled resolution from the list above that will "look like" 2560x1440.

You will get the same screen real estate and same size of everything, just sharper.

However performance will be noticeably worse, in form of a delay after you type something and before it appears on the screen. Of course it will be in range of milliseconds, but you will notice it.

Update: The delay I wrote about above was noticeable on the MacBook I had then (2015 Pro 15 inch without a dedicated GPU). With my current one, a 2018 Pro 15 inch with a dedicated GPU, the delay is not noticeable to me (but of course it's a subjective thing, you may notice it).

I bought one (the Dell P2415Q) so can offer the results...

Scaled options are: 1504x846, 1920x1080, 2304x1296, 2560x1440 and 3008x1692

That is to say, the macbook always outputs 3840x2160, 60Hz as far as the monitor is concerned. It just "looks like" those resolutions in terms of real-estate.

1920x1080 is the default, and other scaled resolutions are given with the warning that "Using a scaled resolution may affect performance".

eg.

Looks like 1920x1080:

Looks like 3008x1692:

Source: My MacBook Pro (Retina, 13-inch, Early 2015), running 10.10.4, 2.9 GHz Intel Core i5, Intel Iris Graphics 6100 1536 MB