Watch the Various Chicks - Mid Strings online guitar lesson by Matthieu Brandt from Guitar Lab: Triads and Hendrixian Double Stops

The triad that can be built on the 4th degree in the Ionian major scale is often not colored in the same manner you would color the I or V chord. Replacing the major third with the fourth note up from the root in the same key doesn’t work for the IV chord. It would become a IV with an augmented fourth added and that's kind of a harsh sound. When the IVth degree chord is extended often a sus4 is added; the resulting mode suggests a 1st degree Mixolydian mode.