The reason the iv (minor 4) sounds so good, is because a phrygian half cadence is a iv to III (minor 4 to major 3) and this cadence occurs a lot in the harmonic modes. The minor four works in any harmonic key, so while you don't have to use a phrygian half cadence, the minor four still works. If you avoid the fifth, natural or sharp, during the melody in the scale while around this minor four chord, you can easily modulate back to the diatonic modes in the same progression. If you're looking for more "out of key" chords, the major third, like I mentioned also works very well :) you can thank the classical composers for these :)