Lets take a few steps back. The speaker builder may crossover their
mid/tweeter in the 3khz - 5khz range because they usually skimp on
the tweeters when planning a design. Very few that make DIY line
arrays use the high end tweeters, instead they use cheaper tweeters
and cross them over higher because they can't hang with a lower
crossover point.

To squeeze out more magic: Find an exceptional tweeter that can be
taken down to 1500hz - 2000hz {as Jim said} and the system not only
will integrate well with almost any midwoofer, but the perception of
better sound and imagining is had.

By taking the full range driver up to 4khz, then crossing to a
tweeter, this won't give you that magic. Sure you can make it work,
but it's not as sweet as the other choice.

I can take an exceptional ribbon or planar tweeter and make a full
array with them and use $1 midwoofers and you will be blown away by
how good the sound is. I can't say the same if the situation is
reversed.

Suppose you did some tests and realized this is the path that indeed
works well. If your tweeters need to operate at 1500-2000hz to
perceive the 'magic', then that implies your midwoofer will operate
up to 1500hz - 2000hz in which case, it's easy to find midwoofers
that can do this job easily with great SQ, and the full range driver
isn't really needed.