I'm thinking about draining my brake fluid, then refilling it. My brake fluid is old. What would happen if I were to use dot4 synthetic brake fluid instead of dot 3 when I replace the fluid? I hear dot4 is better. I don't want to screw anything up.

Nothing will happen. Dot3/4 are relatively the same. Just don't use DOT 5 or higher.
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And GOOGLE the damn properties of DOT4. I'm really getting sick of people asking EASY questions, thats covered ALL over the internet. <span style="font-size:36pt;line-height:100%">GOOGLE IS YOUR DAMN FRIEND! EVERYONE USE IT FIRST</span>. </span>

Well, thank you.

I did a google search site:rdtiburon.com "dot 4" and I didn't find the information I was looking for. I upgraded my clutch fluid to dot4 a long time ago.

In my field I come across many, many things which don't make sense and apply only to a certain piece of machinery. There's tiny engineering differences between every different classification of lubricant. Just because it works on one car, it does not mean it will work on another. This is the reason I ask here.

dot4 works in all dot 3 systems, regardless of 'engineering differences'
dot5 is not compatible (silicone based)

Don't google using the site pre-text, just google the whole database.

My google comment is referring to EVERYONE not just DTN.

As mentioned 4 is fine. It is considered an upgrade for some very small reasons. Just no DOT5!!!