The context says it is at the timing of the RESURRECTION that those who have died in Christ Jesus will "THEN" be "WITH" Him:
1Th 4:13 But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope.
1Th 4:14 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him.
1Th 4:15 For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep.
1Th 4:16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
1Th 4:17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
1Th 4:18 Wherefore comfort one another with these words.
2Ti_4:8 Henceforth there is laid up for me a crown of righteousness, which the Lord, the righteous judge, shall give me at that day: and not to me only, but unto all them also that love his appearing.
Mat 22:28 Therefore in the resurrection whose wife shall she be of the seven? for they all had her.
Mat 22:29 Jesus answered and said unto them, Ye do err, not knowing the scriptures, nor the power of God.
Mat 22:30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
Mat 22:31 But as touching the resurrection of the dead, have ye not read that which was spoken unto you by God, saying,
Mat 22:32 I am the God of Abraham, and the God of Isaac, and the God of Jacob? God is not the God of the dead, but of the living.
Mat 22:33 And when the multitude heard this, they were astonished at his doctrine.
Like them, you do not know the scriptures, nor the power of God, either.