Actually if you look at the discharge charateristic of NiMH batt., you will know more why it slow down and act like a 3cell. When it is fully charged, each cell will around 1.4v (total 4cell ~5.7v) which has a higher top volt. and thus higher top speed.
After a while run each cell drops to a level ~1.1v which act like a fully charged 3cell(~4.3v).
You can refer to my another article for further information:
http://www.tinyrc.com/forums/showthr...threadid=19895