MagentaLine went through a lot of the contingencies. One clarification, though: Loss of #4 hydraulics will not prevent deployment of all the ground spoilers, though manual deployment after touchdown may be preferable to arming them for auto-deployment in some cases.
As others stated, airplane controllability will depend on time of the separation and collateral damage. Loss of the engines and related systems in and of themselves would not cause loss of control. Damage to other systems (LE flaps, bleed air and fuel lines, etc)
For a system summary:
Loss of thrust of #3 & 4 engines: Sufficient thrust from #1 & 2 if starting at reasonable weight and altitude (e.g., NOT right after heavyweight takeoff, still dirty). Once at Flaps 1 and Flaps 1 speeds, should be little problem.
Loss of #3 & 4 generators: Sufficient backup with #1 & 2; no problem.
Loss of #3 & 4 bleed air: IF the separation is between the engine and the pylon valve, and the valve can be closed, air can be supplied to the right wing from #1 & 2. If not, the right Bleed Air Isolation Valve will have to be closed, and Leading Edge Flaps will require alternate (electric) extension and retraction.
Loss of #3 & 4 hydraulic systems: Outboard trailing edge flaps and wing landing gear will have to be extended by alternate means. Wing landing gear cannot be retracted. Primary normal brakes lost, but #1 hydraulic system provides full backup. Some controls (elevators, stab trim, spoilers, etc) will have reduced effectiveness, but airplane is fully controllable. Autopilot 'A' lost; autopilot 'B' still works.
Fuel: If pylon fuel valves are intact and can be closed, crossfeed procedures can be used to maintain balance. Fuel dump is unaffected.