Hello Tosh,
Personally I think "5th generation" is correct.
Th W198 (gullwing & roadster) and the W121 ar not really different generations; they are just different models of the same generation. Both were presented in New York in 1954, the Fluegeltuerer immediately started production run then but the 190 SL took one year longer.
Best,
Achim
(Magdeburg, Germany)