Chrono Trigger/Walkthrough/Chapter 4

Chapter 4: We're Back! (1000 A.D. second time)

You'll pop back into the present at the Millennial Fair, right where you left off. Lucca will leave, and ask you to escort Marle back to the castle. You can buy some more weapons and armor while you're in the area but it probably won't be necessary and you'll find a Lode Sword soon anyway. You can also go back through the gate to 600 A.D. if you really want but it'll be just the same as you left it.

This is your last chance to take care of things at the fairgrounds, see Chapter 1 for a list of things you can do to affect the outcome of your upcoming trial. When you're ready, head over to Guardia forest, 400 years older than when you last left it. Search the bottom right area for a power tab, your third one so far.

As for how things break down at the trial, this is the best that I've been able to determine so far:

1st Juror: Guilty if you claim the fortune tempted you just a little bit OR if you tried to sell the pendant.
2nd Juror: Guilty if you talked to the little girl and didn't bring her back her cat. The outcome of this juror is slightly glitched: if you never talk to the little girl who's missing her cat she won't show up in the trial and that juror will vote Not Guilty. If you talk to the little girl who's missing her cat and don't bring it to her, she'll testify against you and that juror will vote Guilty. The tricky part is that if you talk to her AND bring her the cat, the juror may or may not vote Guilty, I've tried talking to her first and not leaving the screen or leaving the screen, but it doesn't seem too predictable. If you want to be perfectly safe you could never talk to her, and bringing her the cat before talking to her seems to work as well.
3rd Juror: Guilty if you tried to drag Marle away from the candy booth so she utters the "kidnapper" line.
4th Juror: Guilty if you admit the fortune tempted you OR if you picked up the pendant before talking to Marle.
5th Juror: Guilty if you admit the fortune tempted you.
6th Juror: Guilty if you stole the man's lunch.
7th Juror: Guilty if you lied at any point in the trial, either by claiming Marle started it, or by claiming you never stole anything when you did. He doesn't seem to vote Guilty if you lied about not being tempted however when you tried to sell the pendant. Also if you lie twice he still votes Guilty and no one else does.

Anyway, to summarize what you need to do for the best outcome:

Rescue the little girl's cat, although it works just as well if you just never talk to her, or bring her the cat without talking to her first.
Don't eat the lunch.
Don't drag Marle away from the candy booth
Don't try to sell her pendant
Talk to her before picking up her pendant
Tell the truth during the trial, admit you started it and if you stole something admit it
Say you weren't tempted by the fortune at all, not even a little bit. (regardless of your actions)

And if you're going for all guilties, do the exact opposite, and make sure you talk to the little girl with the cat and don't give it back to her. The second juror is glitchy so getting all innocent can be hard, and regardless of what happens at the trial you'll still be sent to prison where the chancellor will change your verdict if he doesn't like it. If you get four innocent jurors, a bare majority, you'll have a gift of one ether waiting in prison. If you get five innocent, you'll get three ethers. Six innocent jurors, 5 ethers, and if all seven vote innocent you'll get 7 ethers.

Challenging Lavos

In a New Game + once again, as soon as you hop back into the present you can step on the right telepod to warp to fight Lavos. Beat him now and you'll see the Frog Ending. You'll only have Crono and Marle to work with so it will be a tough one. The ending basically shows what might have happened if you left 600 A.D. for good at that point, King Guardia got sick not too much later, and Frog might have returned to marry Leene.

Continue to Chapter 5: The Trial (1000 A.D. second time)