it looks like most "deep" platforms listed in the wiki link are ~400m, so this would be 7.2m (assuming the compressibility factor is linear).

either way it doesn't matter for this argument. we're talking about the difference between water and whatever it's replaced with such as "nothing" or something cheap and abundant with similar properties.

also, the water we're talking about is not itself 4 km deep. it may be under 4km of sea water, but the fresh water under the seafloor may be only 500m deep with an equivalent pressure of 4 - 4.5 km.

Volume, not height. At 4km, 100 L of water would be compressed to 98.2 L. Also, that's at 4km depth, not the compression of everything above that point, but rather the compression of things at that point.