I can't help with the comment search, but I can possibly answer questions you may have about that rough area, if helpful?

It depends on what you want to do in your transition. 3/4 and 4/4 are not very different from one another, unlike 6/8 and 4/4 or 6/8 and 3/4. A transition from 3/4 to 4/4 should work reasonably seamlessly without any fancy devices, as you're kind of just adding or subtracting a beat. But if you wanted to make the transition a little more fluid, you can superimpose one on the other for a short period before the change.

For the following, S = strong, W = weak (referring to the beat strength. SS will mean secondary-strong.

In 3/4, your pulse pattern is: S W W | S W W | S W W | etc. (counting ONE two three | ONE two three)

In 4/4 your pulse pattern is: S W SS W |S W SS W | S W SS W| etc. (counting ONE two THREE four | ONE two THREE four).

Ok so for a 3/4 you might have a pulse pattern: S W W | S W W | etc. and you might want to move it into 4/4, so you would introduce the 4/4 pulse pattern against the 3/4 pulse pattern, say, in one instrument.

I1 3/4|| S W W | S W W | S W W | S W W | S W W | S W W | 4/4 S W SS W | S W SS W | etc.

I2 3/4|| S W W | S W W | S W W | S W SS | W S W | SS W S | 4/4 S W SS W | S W SS W| etc.

In that example, instrument 2 introduces the "polymetre" of 4/4 against 3/4 facilitating a slightly smoother move into 4/4.

Notice that in the final bar before the 4/4 change there will be a "double accent" where the final strong pulse of the 3/4 has a new downbeat in 4/4. To get around this you simply disregard that beat structure a bit (I.e SS W W | 4/4 S W SS W) or make the transition long enough to have the full length of 3:4 (12 beats).

There are other ways to do this, but that is one of the easier ones. Is that what you are after?

Short answer is "yes", long answer is: "kinda".

Well, simply changing time signature isn't necessarily metric modulation. Metric modulation is changing between tempo and/or time signature where one fundamental pulse becomes a new, different, fundamental pulse. For example: an 8th note duplet becomes an 8th note triplet (1.5x faster, approximately) and that triplet then becomes the new 8th note duplet at a new tempo that is 1.5x faster than the original. Shifting from 4/4 to 3/4 at the same tempo isn't really something that needs/is metric modulation, because it is just losing (or gaining) a quarter-note impulse, but if you're really adhering to the difference in "feel" (few people actually do this), then adding the "feel" of one or the other by a technique such as polymetre, or polyrhythm, facilitates the change. So there are different metric devices that facilitate different types of changes in different ways. Hence my vague answer.