In guitar, G position is referring to the CAGED G position. Please see 3rd one in the diagram below.
The root note for G position would start on the sixth (lowest) string and the scale notes would fall on the frets above inclusive.
When scaling over IV chord, you play using the above G position but use the Lydian mode for the key in the G position. That is, the starting or ending or both (depending, I think) of the scale notes would be the 4th diatonic note, played in the G position.
For example, you play a song in the key of A.
For the G position, the root note would start on the 5th fret 6th string.
To scale over the IV chord in that G position for key of A, you would not want to start or end on the root note of the G position. Instead, you would start or/and end 4th diatonic note, which would be D note, 5th fret 5th string, scaling over the notes in A scale, which is basically Lydian mode for A.