I also framed my 2 4x8 tables with 2x4s and covered them with 7/16" OSB. I built them close to 20 years ago and they seem to be holding up fine. The half table I made is framed with 1x4s and topped with OSB also. I am reluctant to stand on that section because there are no legs for it; it is just wood screwed to the other tables.
J White