#4.
A spell that takes 1 round to cast is a full-round action. It comes into effect just before the beginning of your turn in the round after you began casting the spell. You then act normally after the spell is completed.
It then goes on to say that spells with casting times longer than 1 round require the use of full-round actions for each round throughout, without interruption.
EDIT: As DM_Blake points out below, #1 seems to be a valid choice as well. I had indeed forgotten about that rule, as splitting full-round options has never come up in my games.
So I assume you mean that you move in one round (let's say the first round) and then begin casting a One-Round spell that finishes on the following round (the second round). Is that right?
Casting as a full-round action requires you to use a full round to cast the spell, as well as continuing your concentration until right before your next round begins. Splitting the actoin between two rounds DOES change this so that it requires your standard action in each round.
This is because of this oft-forgotten tidbit:
Start/Complete Full-Round Action
The "start full-round action" standard action lets you start undertaking a full-round action, which you can complete in the following round by using another standard action. You can't use this action to start or complete a full attack, charge, run, or withdraw.
So it works like this:
Round 1:
Move action to move or do whatever.
Standard action to begin casting the spell.
End your turn but you are still casting and could be interrupted.
Round 2:
Standard action to finish casting.
You still have your Move action to do whatever you want.
End your turn.