There is a bug in FSX that means that if your last flight or the last aircraft you flew had its engine(s) shut down by either the mixture control on piston engines or by the fuel cut-off levers/switches or by using CTRL+SHIFT+F1 keys then the fuel supply remains cut-off despite setting the engine fuel cut-off control to ON in the DC-8. The way around this is to press CTRL+SHIFT+F4 keys BEFORE you start the engines, the engines can then be started as per the manual instructions and the engine fuel cut-off levers will function as designed for all the engines.

NOTE! The procedure as detailed in the manual for setting the fuel control and cross-feed levers for starting engines 3 and 4 MUST be followed exactly otherwise the engines will not start. 

IMPORTANT! Be aware that the engineers fuel control panel has five sets of levers. Engine 1, Engine 2, Centre Tank, Engine 3 and Engine 4 in that order. Please ensure that you are not confusing the Centre fuel tank controls for Engine 3 controls as doing so will not allow the engines to start.