How to write dashes in "a 2-4-room-apartment"?

I believe that the most typographically correct method of spelling it out while using numbers would be a 2- to 4-room apartment. (Unfortunately my usual source for such things is silent on this type of matter, but I swear I've seen this in a reasonably authoritative location someplace.)

My logic for this is that 1) using multiple hyphens or dashes in close proximity for different purposes is confusing, so it's better to spell out one of the potential hyphen points; the "to" is the obvious choice since you're going to pronounce it "to" even if it's written as "-", and 2) you would ordinarily write "2-room" and "4-room", so keeping the 2's hyphen with it is the logical way to show that the 2 is still intended as a prefix for "room".

The convention says to use an en dash to indicate the range and a hyphen to complete the phrase if you want to use numerals:

a 2–4-room apartment

That assumes that you have some control over the actual printing/display. Using all hyphens and the word to will probably be more clearly understood by your readers:

a 2-to-4-room apartment

It isn't typographically correct (you would need to spell out the numbers to make the hyphens "correct") but it gets the message across clearly -- and clarity is the most important part of any communication.

2-4 room apartment will do fine. I wouldn't bother to use an en dash; some people will say you should, but it's trivial in this context.

If you specifically wanted one with three rooms you could write 3-room, but I don't think there's any need to have a separator even there.

I certainly wouldn't include the word to. It's an advertisment, not an essay!

The use of the hyphen between words is a handy way to make them a compound adjective. The difference between 'Her suit was made-to-measure linen of the highest quality' and 'Her suit was made to measure linen of the highest quality' is not trivial, I think.

I'd suggest that defends the use of - in fact the need for - the hyphen both in 3-room and in 2–4-room. So I go with en-dash + hyphen - like Stan.