Until a few years ago I played 4...Qh4?! vs. the Scotch but finally came to my senses, switching to 4...Qf6.  Interestingly this lines gets almost no coverage in books which however give coverage to 4th move alternatives such as 4...Bb4.  

Typically 4...Qf6 transposes to variations that begin 4...Bc5 but there can be lines with independent significance, including 4...Qf6  5. Nb5 and lines after 5. c3 or 5. Be3 with ...Qg6 before ...Bc5.

But the lines with an accelerated ...Qg6 seem to be easily enough countered with Nb5, so I see nothing better against c3/Be3 than to steer back into ...Bc5 lines.  But not willing to entirely give up on plans with ...Qg6 I realize there is an alternative to the more theoretical sixth move 6...Nge7 in the line 4...Bc5  5. Be3 Qf6  6. c3, and that alternative is 6...a6!?

This allows for ...Qg6 without fear of Nb5 and any loss of tempo can be somewhat justified by White's less than optimal formation in that Nc3 is precluded.