Maryland Route 408 is a state highway in the U.S. state of Maryland. Known as Mount Zion-Marlboro Road, the highway runs 5.62 miles (9.04 km) from MD 4 at Waysons Corner east to MD 2 and MD 422 in Lothian. MD 408 is the old alignment of MD 4 in southern Anne Arundel County. The highway was constructed in the late 1910s and became the easternmost part of MD 4 in 1927. The highway received its present designation in the mid-1960s when MD 4 was rerouted south into Calvert County. MD 408's western end was relocated when MD 4 was upgraded to a freeway through Waysons Corner in the early 1990s. MD 408 was also applied to the old sections of MD 4 between Andrews Air Force Base and Upper Marlboro in Prince George's County. The number was assigned after the MD 4 freeway was completed in the mid-1960s. The Prince George's County portions of MD 408 were transferred to county maintenance in the late 1970s except the section through Upper Marlboro, which became MD 725.

Maryland Route 4 is a state highway in the U.S. state of Maryland. The highway runs 64.85 miles (104.37 km) from MD 5 in Leonardtown north to Southern Avenue in Suitland at the District of Columbia boundary, beyond which the highway continues into Washington as Pennsylvania Avenue. MD 4 is a four- to six-lane highway that connects Washington and communities around Interstate 95 (I-95)/I-495 with southern Prince George's County with southwestern Anne Arundel County. The highway is the primary highway for the length of Calvert County, during most of which the route runs concurrently with MD 2. MD 4 also connects Calvert and St. Mary's counties via the Governor Thomas Johnson Bridge across the Patuxent River. The highway connects the Southern Maryland county seats of Leonardtown, Prince Frederick, and Upper Marlboro.