Why were more people exiled in his 23rd year?
Rashi 1 citing Seder Olam 26: That year he conquered Tzor, and overran all the Yehudim in Amon and Mo'av and around Eretz Yisrael.
We fixed the text of Rashi based on Radak and our text of Seder Olam.
Who are included in the 4,600?
Radak: This is the total of those mentioned in verses 28-30 (3,023 1 in Nebuchadnetzar's seventh year, 832 in his 18th year (the year of the Churban), and 745 five years later).
Malbim (Melachim II, 24:14): In Daniel it says that when Nebuchadnetzar conquered Yehoyakim, he asked to bring children from Yehudah and the kingship; only here it reveals how many. The 4,600 excludes the exile of Yehoyachin.