The fourth mode of the Melodic Minor Scale is called the Lydian Dominant. If you're hip with your theory you know that Lydian means the #11 (#4) is present, and Dominant means there's a b7 about too - which perfectly describes this scale!

It's a super hip kind of sound used by modern Blues and Jazz players.

The scale formula for the Lydian Dominant is below, it describes the interval distances between each note of the scale. Note the 4 Tones in a row that help give this scale it's unique quality.

	T		T		T		S		T		S		T
1		2		3		#4		5		6		b7		8

T = Tone [whole step] • S = Semitone [half step]

Comparison With Major Scale

Scales are always compared to the Major Scale to find the intervals used.

	I	II	III	IV	V	VI	VII	I
C Major Scale	C	D	E	F	G	A	B	C
C Lydian Dominant	C	D	E	F#	G	A	Bb	C
Equation	1	2	3	#4	5	6	b7	1

As the name suggests it's easiest to think of this as a Lydian Mode with a flat 7th (b7), though to my 'use' of it I'd be more inclined to think of it as a Mixolydian #11 (#11 and #4 are the same note, the naming is just about context).

Chords And Extensions

Because we're thinking modally here we're looking at the individual chords you might use this scale over not the Chords In The Key of the Melodic Minor Scale.

This scale is played over Dominant chords, specifically a 7#11 but really you can use it over any 7/9/11/13 - remember that the #11 is the same as a b5 which is the 'blue note' so it's almost like adding that into a common Mixolydian Mode and taking away the natural 4 which is a duff note to play on a 7 chord anyway! I'd say this is the best scale to play over an unaltered Dominant!