1 Cor 10:4 may seem to imply that Jesus preexisted to some. However, in this context, "Christ" is used as a typology, referring to what God has anointed (to provide salvation). This pertains to spiritual food and drink (1 Cor 10:3-4)
In ancient Israel, the rock was what God anointed to provide salvation. (1 Cor 10:4) Now Jesus is whom God has anointed to provide salvation.
Acts 10:37-38 (ESV)
you yourselves know what happened throughout all Judea, beginning from Galilee after the baptism that John proclaimed: how God anointed Jesus of Nazareth with the Holy Spirit and with power. He went about doing good and healing all who were oppressed by the devil, for God was with him.
Types of Christs were provided by God in the past. Now God has provided a new and better savior (means of salvation), his begotten son, Christ Jesus.
Heb 5:5 (ESV)
So also Christ did not exalt himself to be made a high priest, but was appointed by him who said to him,
“You are my Son, today I have begotten you”;