Four thousand - Four thousand "men," besides women and children. See Matthew 15:38. See this passage explained in the notes at Matthew 15:32-39.

And they that had eaten were about four thousand,.... That is, men, besides women and children, as Matthew observes; See Gill on Matthew 15:38.
and he sent them away; some that came dumb, with their speech, and deaf, with their hearing; others that were maimed, with perfect healing of their wounds, and with their limbs sound and whole; others that came lame, he dismissed leaping; and others that were blind, with their sight restored to them, and all of them full.

And they that had eaten were about four thousand: and he sent them away--Had not our Lord distinctly referred, in this very chapter and in two successive sentences, to the feeding of the five thousand and of the four thousand as two distinct miracles, many critics would have insisted that they were but two different representations of one and the same miracle, as they do of the two expulsions of the buyers and sellers from the temple, at the beginning and end of our Lord's ministry. But even in spite of what our Lord says, it is painful to find such men as NEANDER endeavoring to identify the two miracles. The localities, though both on the eastern side of the lake, were different; the time was different; the preceding and following circumstances were different; the period during which the people continued fasting was different--in the one case not even one entire day, in the other three days; the number fed was different--five thousand in the one case, in the other four thousand; the number of the loaves was different--five in the one case, in the other seven; the number of the fishes in the one case is definitely stated by all the four Evangelists--two; in the other case both give them indefinitely--"a few small fishes"; in the one case the multitude were commanded to sit down "upon the green grass"; in the other "on the ground"; in the one case the number of the baskets taken up filled with the fragments was twelve, in the other seven; but more than all, perhaps, because apparently quite incidental, in the one case the name given to the kind of baskets used is the same in all the four narratives--the cophinus (see on Mark 6:43); in the other case the name given to the kind of baskets used, while it is the same in both the narratives, is quite different--the spuris, a basket large enough to hold a man's body, for Paul was let down in one of these from the wall of Damascus (Acts 9:25). It might be added, that in the one case the people, in a frenzy of enthusiasm, would have taken Him by force to make Him a king; in the other case no such excitement is recorded. In view of these things, who could have believed that these were one and the same miracle, even if the Lord Himself had not expressly distinguished them?
Sign from Heaven Sought (Mark 8:10-13).