2:4 filled with the Holy Ghost. Although the Spirit’s baptism and filling occurred simultaneously in this initial manifestation of His power, the two are separate ministries. All believers are baptized by the Holy Spirit into the body of Christ once for all at the time of conversion (I Corinthians 12:13). They can, however, be filled with the Spirit on more than one occasion and, in fact, are exhorted to “be [continually being] filled with the Spirit” (literal reading of Ephesians 5:18). Note, for example, the repeated fillings recorded in Acts 4:8,31; 9:17; 13:9.
2:4 other tongues. These were “other tongues,” not “unknown tongues” or “ecstatic tongues.” Except when referring to the actual physical organ, the word “tongue” in the New Testament always refers to a language. In this case, the tongues are the actual languages of the different nations listed in Acts 2:9-11, as made obvious in the context.