I don't know if your kids are like mine, but if they are, they might sometimes need to work themselves up into a tantrum. I mean, sometimes the premise is so ridiculous they have to spend some quality time even convincing themselves it's worthwhile to cry about.
In these instances, my child will ask me a seemingly innocent question. I will answer. The question then comes again, a little more urgently. I will pause, see the hole into which I'm about to fall, and attempt to answer using slightly different words. Now, sometimes I'm blessed with a warning, "you just don't understand me!" after which I can encourage the use of other words. But many other times, the answer simply is one my child doesn't want to hear or cannot believe. Or neither. Sometimes she just wants to cry.
With four year olds, delicacy and tact will often get you nowhere. When my child is fixing to cry like this--when I can see the cogs in her brain turning rapidly, edging her emotional state to the brink--I stop it with a blunt, to-the-point question that brings to the forefront what is actually going on (because my kids aren't really doing this on purpose. They only know that they feel a certain way and those feeling, to their subconscious, demand a certain kind of relief.) I ask them, "Are you going to cry?"
"Mom, why are there rainbows?"
"Well, after a storm, the sun sometimes shows through the clouds and the light is reflected and refracted, splitting it up into all the colors and showing them in the sky."
"Mom. Why are there rainbows?"
"When the sun comes out after the rain, sometimes it makes a rainbow."
"You just don't understand me."
"Are you going to cry?"
Asking the question gets rid of the undercurrent as the child recognizes what she is accidentally doing to herself. And without that undercurrent, it's easier for us to reach an agreement about rainbows without either of us getting emotional.
"Mom, can I have a candy?"
"No, how about some string cheese?"
"No, I want a candy. Can I have a candy?"
"You just had a candy. You have to wait before you can have another one. Sure you don't want string cheese?"
"I want candy!"
"You can't have candy right now. That's not going to change. Are you going to cry about it?"
Addressing the issue directly gives the child the choice to cry or not, whereas before, she really didn't have a choice. She was going for it, maybe without realizing it. When you put the outcome out on the table like that, your child can then assess her options. Usually, mine will make the decision, no. I'm not going to cry about this. But they wouldn't have been able to do that unless I asked them.
In my opinion, this just helps them hone their own mental processes. Soon enough (one would hope), their minds would ask this question to themselves with lightning quick speed, and the decision, then, that they make to make a fuss or not, will be a true decision.
But at four, they're just going along with the tides. Might as well help them swim.