The magazine must have been reloaded after the 4 shots to NB. 
Yes the gun was reloaded after shooting Nevill in the kitchen, it was likely only partially loaded so that he could quickly be shot before he woke up and caused more problems. Then with him the dead the killer had extra time to fully reload before going back upstairs to finish the job.
There are 3 distinct shootings and some slight variations possible.
I. Prior to the kitchen episode
II Kitchen Episode
III Post Kitchen episode.
The kitchen episode had 4 shots there is no variation possible. It is 1 and 3 that have variation possible.
IA) The gun was initially loaded with 11 rounds all 7 bullets into June and 4 into Nevill
IB) The gun initially had 10 rounds 6 bullets into June and 4 into Nevill
IIIA) Returned from the kitchen with 10 rounds, 8 into the boys and 2 into Sheila
IIIB) Returned from the kitchen with 11 rounds, 1 between June's eyes, 8 into the boys, 2 into Sheila
Not that it is theoretically possible for the boys to have been killed before Sheila. We know the 10 rounds were used on them but can't be sure the boys were killed first. If Sheila was already out of her room she had to be dealt with before the boys. If Sheila was still in her room then it makes more sense to kill the boys first and then her. Obviously if one wants to assert she was the killer she had to die last.