As you say the Christians believe the Holy Spirit in Acts of the Apostles 2:1-4 to be the same Holy Spirit Christ talks about in the gospel of John.
I agree the Holy Spirit descended upon them at Pentecost but that is a separate event from the Comforter being sent later.
At Pentecost they were empowered to go out and proclaim the gospel Jesus had taught them. They weren't being told something new that they couldn't bear to hear when Jesus was still alive. That doesn't make sense. What makes more sense is the comfort refers to a new Revelation from God according to the more advanced capacity of people in the future to receive that message.
Lets look at those verses in the Gospel of John more closely .
John 16:12-14 I have yet many things to say unto you, but ye cannot bear them now. Howbeit when he, the Spirit of truth, is come,
he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come. He shall glorify me: for he shall receive of mine, and shall shew it unto you.
So if it was 'all truth' they could not near to hear, why were not ready for 'all truth' at that time? That description best fits Baha’u’llah who revealed 'truths' that clearly the disciples of Christ were not ready for at that time. Of course 'all truth' can't literally be all truth so meant relative to what Christ taught.
Jesus was “a Comforter” because Jesus brought the Holy Spirit. Muhammad and Baha’u’llah were other Comforters and also had the Spirit of truth.
John 14:16 And I will pray the Father, and he shall give you
another Comforter, that he may abide with you for ever;
John 14:17 Even the
Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
It is the Comforter who fulfils these verses in John, not the Holy spirit at Pentecost spoken in Acts.
John 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name,
he shall
teach you all things, and
bring all things to your remembrance, whatsoever I have said unto you.
John 15:26 But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father,
he shall testify of me:
John 16:8 And when he is come,
he will reprove the world of sin, and of righteousness, and of judgment:
John 16:13-14 Howbeit when he, the Spirit of truth, is come,
he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come. He shall glorify me: for he shall receive of mine, and shall shew it unto you.
Muhammad and Baha’u’llah did all these things as Jesus promised.