DOT 4 will likely have slightly higher boiling points - which is of interest if you will be driving some place with long downhills. The FMVSS only mandates the minimum boiling points so some DOT3 will have higher BP than DOT4s.
A decent summary and link to the Brake fluid FMVSS DOT standards here:
http://www.stoptech.com/technical-support/technical-white-papers/brake-fluidHowever Valvoline makes a brake fluid that meets both DOT 3 and 4. According to the MSDS, it does not contain Borate estor (as is typical in DOT 4s according to the linked article). My personal experience has been that it does not absorb moisture as quickly as Castrol's LMA DOT4. I bleed the brakes every year - usually in the spring or early summer.
