I'd say the difference is about 1-2lbs or .002"

I'd check that Vanish spool again. The stated pound test and diameters of the smallest sizes of Berkley Vanish are as follows:
  • Vanish 2lb - .006 - Equivalent in diameter to Frog Hair .006 or 5x with a stated breaking strength of 4lbs
  • Vanish 4lb - .007 - Equivalent in diameter to Frog Hair .007 or 4x with a stated breaking strength of 5lbs
  • Vanish 6lb - .009 - Equivalent in diameter to Frog Hair .009 or 2x with a stated breaking strength of 8lbs
  • Vanish 8lb - .010 - Equivalent in diameter to Frog Hair .010 or 1x with a stated breaking strength of 10lbs
So based on this comparison Vanish 4lb is equal to Frog Hair 4X, not 6X but is one pound weaker.

This comparison is asked about lot and the results are always the same. If you believe stated strength and diameter measurements; tippet material is always stronger for the same diameter than fishing line. It is also more supple in most cases.

Whether or not that matters for the type of fishing you do is up to you but the bottom line is, the similarity ends when you compare diameters to strength.