I provided dimensioned drawings of the 150, 300, and 400-gallon Douglas low-drag external tanks here: http://tailspintopics.blogspot.com/2011/07/douglas-low-drag-external-fuel-tanks.html
The tanks and fins were standardized but the number and location of the fins varied with the specific location on a particular type.
The 150-gallon tank was common on the A4D-1 and early A4D-2s.
Because the tanks were very similar in shape, you have to look carefully to see which one is installed. In the case of the A4D, the clues are a somewhat smaller diameter relative to the airframe and more obviously, how far the tank extends behind the wing trailing edge and to a lesser degree, ahead of the wing leading edge.
Here is an example of a single 400-gallon tank on an A-4 and one with two 300-gallon tanks. Note that the 300-gallon tanks do not have fins, which is not unusual, but the forward location of the national insignia on an A-4E is.
I've seen questions about which kits have accurate external tanks so I did a quick survey of some of the 1/72 kits in my stash, comparing their 300-gallon tanks to the drawing. In rough order of acceptability:
Esci A-4 (A4D): Excellent
Hasegawa A-1 (AD): Almost perfect. Rounding the nose a bit provides almost exact length; the fins are a bit small but not objectionably so.
Tamiya F4D: Very slightly oversized, rounding the nose would be appropriate.
Airfix F4D: Very slightly oversized.
Airfix S-3: Basic size and shape okay; the fins need to be cut down. (The basic shape of the airframe is also better than the Hasegawa kit, e.g. the engine nacelles aren't circular.)
Hasegawa EA-6B: Basic tank okay but fins slightly undersized.
Fujimi A-6: Length okay but diameter a bit undersized and too tapered; fins good.
Hasegawa S-3: Too short by about 13" and forward edge of fin angled downward too sharply.
Fujimi A-7: Too short by about 31" and terrible fins
Airfix AD: Somewhat undersized (too tapered/conical) and fins too small.
Hasegawa A-4: Definitely undersized; could be sectioned to create the 150-gallon tank.
Matchbox F3D: 150-gallon size but forward part of tank is too tapered, conical.
Hasegawa F9F-8: 150-gallon size but too small in maximum diameter; taper down in diameter from mid section both forward and aft is too conical; and the fins are not correct
For some aftermarket options, see http://tailspintopics.blogspot.com/2013/08/things-under-wings-douglas-low-drag.html
Comments, additions, and corrections welcome.