You're right - the Fourth Street Bridge is different from the Johnson Street Bridge, but it was the City that made the initial comparison, using it to argue that refurbishing the Johnson Street Bridge would be too expensive and difficult. (See
city webpage, powerpoint presentation
link for 4/23 at bottom of page - slides 7 & 8 on powerpoint presentation.)
Here's a good photo of the Fourth Street Bridge:
As for length / sizes:
According to the website
Structurae, the Fourth Street Bridge (completed 1917) has a main span of 28.71 m and a total length of 62.51 m. (See
this page.)
The Third Street Bridge (Lefty O'Doul) (completed 1932) has a main span of 43.62 m and total length of
89.92 m. (See
here.)
Our Johnson Street Bridge (completed 1924) has a "length of movable section" of 45.11 m (unfortunately, the page does not give the data in main span/ total length units, as for the two S.F. bridges). (See
here.)
I'm having some difficulty in finding apples-to-apples comparisons. But the Lefty O'Doul-Third Street Bridge is clearly a lot bigger/ longer than the Fourth Street Bridge. At the same time, I don't know whether "movable section" (in JSB) equals "main span" of the others. If so, then JSB is right up there with Lefty O'Doul, though.