Many of the children of these families do not remain Jewish, but in fact assimilation was already a generation old. To actually be a 3rd or 4th generation Reform Jew would require that one's ancestors, living in the 1800s, be Reform Jews. The major growth of Reform Judaism has been in the 20th century. So while it is true there are few 3rd or 4th generation Reform Jews numerically, they are not a large number primarily due to the heavy immegration in the early 1900s. The majority of today's Reform Jews have ancestors who were O or C, but primarily because there was a limited pool of R on whom to draw.
Anecdotal evidence suggests that number is large.