When God gives us life, He does not take it away. "He has set eternity in the hearts of men" it says in Ecclesiastes 3:12, and although we have to die physically (because "the wages of sin is death" and we are all sinners), Ecclesiastes chapter 12 explains that, at physical death "the spirit returns to God who gave it". Then we have to account to God for how we lived in the body (that then rots in the grave).
God has appointed a day in which all the dead will be resurrected, to stand for judgment, for the accounting. Then their departed spirits will be 'clothed' with those resurrection bodies that will be fit for the final, eternal state, whatever that turns out to be. Revelation 20:7-15 describes that future time.
The eternal state of satan and the demons is stated in those verses; "They will be tormented day and night for ever and ever." See also Revelation 14:11. But eternal life in joy and bliss will be the portion for those not aligned with those evil forces. The connection with Acts 4:12 is simply this - those who align themselves with Jesus Christ, putting total faith in His name to be saved, will be saved for all eternity because of the grace of God in saving sinners who repent in faith.