For both actions in the checklist, there is a note saying "Action is irreversible". But is it really irreversible for the system with the functioning hydraulic system(which in this case was the gear that worked properly). After all, the hydraulics for that system are still working.
In theory, yes. Is it advisable? probably not. An attempt at normal retraction may do further damage. In this situation, there may have been enough #4 hydraulic system pressure to unlock the left wing gear downlocks, but not retract them. This might prevent that gear from relocking (with the assist of gravity) on re-extension.
The question I have is: should there be a visual inspection done of the aircraft before takeoff? Do the flight or ground crew have to look for such damage? Being as it is under the wing but still plenty of headroom it would have been visible for anybody looking for it.
Basic inspections are carried out during transits. If it was broken before (I'm not saying it was before or after), would it have been spotted? It involves looking into the wheel well. The inboard doors are closed at this point, so viewing this device involves climbing into a very greasy and potentially dangerous wheel well. The maintenance manual has a very long list of to-do's before climbing in there.
This "small clip" is actually quite large and robust and heavily covered in green grease for (intended) slipperiness. What damage you might have spotted during a walkaround is debatable. As always, inspections are dictated by the manufacturer and aviation authorities. For example, you don't need to tighten the bolts holding on the wings after every flight
Genuine question from a non-aviator, having just landed safely at 150kts or whatever the speed was, why is it deemed unsafe to slowly taxi the aircraft far enough clear of the runway so the airport can remain open?
The 747-400 braking system has two hydraulic systems as backups,
assuming the rest of the aircraft has power (electrical, pneumatic, etc). The #4 Hydraulic system is particularly important, however, as it repressurises the emergency braking system (which is used for parking, too). Taxiing usually involves turning or accelerating/decelerating an aircraft. This might tip over the aircraft (depending on CG and the gears not extended). Also, the brakes/tyres might be excessively hot due to manual braking and only 3 gears doing the braking. It seems to me like a good idea to stop the aircraft and let the fire crews to assess the exterior damage.