Most Frigates were fifth and sixth rates. After they revised the rating system is when those few remaining 50s became Fourth Rates. But the Napoleonic era saw a major decline in fourth rates use. I think there were more than two though because those were the ships they tried mainly using against the Americans (unsuccessfully =D) in 1812.
The abnormal use of calling fourth rates Frigates probably stems from American super frigates which in use never had less than fifty guns. I don't think Fourth rates were use much in the line in the late 1700s minus the one at the battle of the Nile.