The simple answer here is, almost certainly because Peter and John (at least!) were present on both occasions, see acts 4:23 and Acts 2:1, 14, etc.
Given this, it is almost certain that, very likely, several other disciples/apostles were also present on both occasions.
Therefore, there were people present on both occasions in Acts 1, 2 and Acts 4.
It is commonly understood that the book called, "Acts of the Apostles" should have been more accurately called, "Acts of the Holy Spirit" because of the way it is written - The functioning of the Holy Spirit is quoted very often.
However, in addition to the continuous work of the Holy Spirit in the life and work of the Apostles, there ...
In this prayer, the disciples were pointing out one of the most important things to remember when persecution comes: The persecution was not really against them, but against the Lord.
It is also very important to take special notice of the contents of this prayer. This is the first persecution (or censorship) that the church ever experienced and their first ...
I make the assumption that the early Church was expecting and waiting for God to intervene,
More expecting and less waiting.
i.e., there was an element of persevering in communal prayer with expectancy, waiting for a clear response from God to their prayers before taking action on their own.
God and the apostles were acting simultaneously. The early ...
Before Joshua died, he gave a similar lengthy speech to the Israelites outlining their history in Joshua 24.
Stephen might have pattern his speech after Joshua's. Initially, he tried to appeal to their strong sense of being the same people and shared the same history.
Alas, history itself showed that Act 7:
51 You stiff-necked people with uncircumcised ...