20legend:
to answer your query more accurately, looking at the image(A343 on TDown),
this particular crew still had the nose extremely high for thrust reverser activation!
was it not for the spoilers all being extended, one could mistake the image for a take off, with the nose so high.(thrust reversers deploying, excluded)
as for the different stages of deployment,(#3 full open & #4 just cracking) this is also dependant on how synchronously the pilot selects all 4 levers, plus some doors on some engines may deploy/stow at diffferent rates, due friction/winds etc.
NOTE: no reverser can/should be operable until the landing gear has contacted the ground, plus time delay, couple of seconds, however this can be activated before all 4 wheels on each main gear, touch the tar.
no eng power can be applied in reverse until all 4 doors on each respective engine is fully open, & furthermore, #1 and 4 engines cannot be powered up until both engines have all 4 of their doors fully open, this to prevent an asymetric yaw situation, should 1 of the outboard engs not open fully, and the opposite goes to max reverse power! bad situation.
outboard engines have a far greater yaw effect on an aircraft due to the distance from the centre turning point, & the lever-arm length.
As already mentioned, modern reversers only deflect the fan or low pressure air, which on this particular engine is approx. 65-70%. small fan.
trust this clarifies, an unusually captured shot!