They are the angels that Peter refers to in 2 Peter 2:4-5 (WEB):
(4) For if God didn’t spare angels when they sinned, but cast them down to Tartarus, and committed them to pits of darkness to be reserved for judgment;
(5) and didn’t spare the ancient world, but preserved Noah with seven others, a preacher of righteousness, when he brought a flood on the world of the ungodly;
The angels are "spirits" (spirit beings, not human beings) that are imprisoned in darkness (in Tartarus, not Sheol) because they sinned by interfering in the gene pool of the human race, as described in Genesis 6:
(2) God’s sons {angels} saw that men’s daughters were beautiful, and they took any that they wanted for themselves as wives.
(4) The Nephilim were in the earth in those days, and also after that, when God’s sons came in to men’s daughters and had children with them. Those were the mighty men who were of old, men of renown.
The Nephilim were the offspring of the angels (taking on flesh bodies) and human women. This is why Peter writes, 1 Peter 3:19-20 (WEB):
(19) in which he also went and preached to the spirits in prison,
(20) who before were disobedient, when God waited patiently in the days of Noah, while the ship was being built. In it, few, that is, eight souls, were saved through water.
and Jude also mentions them, Jude 1:6 (KJV):
(6) And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
Peter was referring to these fallen angels who sinned in the days of Noah, and were largely responsible for God wiping out the whole of mankind, because He needed to eliminate the corrupted human genes. Noah was "perfect in his generations" (Genesis 6:9, MKJV), i.e. his DNA was pure human and had not been corrupted by the disobedient angels (note it says "generations", not "generation", and "perfect" is translated from the Hebrew word
tamiym which can mean without blemish, undefiled, unimpaired).
So these spirits are alive (which is why they are in prison - dead people don't need to be imprisoned; they can't go anywhere), but they are prevented from materialising as men. Jesus, in his resurrected spirit being body, went to preach to these angels that were imprisoned in Tartarus - he didn't go to Sheol to preach to dead humans, as that would be pointless because the dead are unconscious and can not think about anything (when we die our "thoughts perish" - Psalms 146:4).
I think 1 Peter 4:6 (WEB):
(6) For to this end the Good News was preached even to the dead, that they might be judged indeed as men in the flesh, but live as to God in the spirit.
is referring to (4) - Christians who had already died when Peter wrote that verse. The passage is talking about suffering for righteousness' sake (WEB):
1 Peter 3:8-9
(8) Finally, be all like-minded, compassionate, loving as brothers, tender hearted, courteous,
(9) not rendering evil for evil, or insult for insult; but instead blessing; knowing that to this were you called, that you may inherit a blessing.
1 Peter 3:14
(14) But even if you should suffer for righteousness’ sake, you are blessed. “Don’t fear what they fear, neither be troubled.”
1 Peter 3:16-18
(16) having a good conscience; that, while you are spoken against as evildoers, they may be disappointed who curse your good way of life in Christ.
(17) For it is better, if it is God’s will, that you suffer for doing well than for doing evil.
(18) Because Christ also suffered for sins once, the righteous for the unrighteous, that he might bring you to God; being put to death in the flesh, but made alive in the spirit;
1 Peter 4:1
(1) Therefore, since Christ suffered for us in the flesh, arm yourselves also with the same mind; for he who has suffered in the flesh has ceased from sin;
1 Peter 4:3-6
(3) For we have spent enough of our past time doing the desire of the Gentiles, and having walked in lewdness, lusts, drunken binges, orgies, carousings, and abominable idolatries.
(4) They think it is strange that you don’t run with them into the same excess of riot, blaspheming:
(5) who will give account to him who is ready to judge the living and the dead.
(6) For to this end the Good News was preached even to the dead, that they might be judged indeed as men in the flesh, but live as to God in the spirit.
Despite returning blessing for evil, the Gentiles (non-Christians) judge Christians as strange, and speak against them falsely as evildoers (much like antisemitism, in which people for no logical reason hate Jews). 1 Peter 4:6 is saying that Christians that are now dead, who were wrongly judged and persecuted while they were alive, and may even have been killed simply because they were Christians, despite that judgement by men, according to God, who judges our spirit (intentions) they are judged as worthy of eternal life. Perhaps the TLV translation is a bit clearer:
(6) For this was the reason the Good News was proclaimed even to those now dead, so that though they are judged in the flesh before humans, they might live in the Ruach before God.