Whatever you do, don't ride Figment first (or at all, really). That ride terrified my 4 year old and he wouldn't go on anymore dark rides the rest of the trip.
I think this is a perfect illustration of how each situation is different and depends on the individual child. I'm not in any way, shape or form saying that it's crazy for your 4 year old to be frightened by the dark part of Figment. But it could have been any number of factors, as a lot of rides at WDW have darkness at some point (think Stitch). Figment was one of the "non-scary" rides we did when our kids were 4, as they weren't scared of it. But through the eyes of a 4 year old there could be any number of things that they see/hear before a ride goes dark, that when in combination with the onset of dark, could be scary for them. Again, not saying it's better or worse than your situation, just every kid is different.
Same will hold true for TT.
I also think parenting style has a lot to do with it too. Again, not any judgments, just recognizing the fact that every parent handles these things with their kids differently. Some parents will avoid rides their kids say they are scared of. Others will push them to go on. I tend to push them to go on a ride. I'm of the mindset that they won't actually know if they like it or not until they try it. I'm not worried that they'll get hurt, and the worst that will happen is they don't like it. I can't tell you how many rides I've dragged my daughter onto crying and grasping at my arm in fear, only to have her get off the ride giggling...SM, RnR, ToT, etc... In fact, SM and RnR are probably 2 of her favorite rides. For RnR, even after the first time she rode it, the next few she insisted on sitting next to me. She still had a few tears in her eyes and held my hand. Now, she couldn't care less if she's next to me or one of her brothers. That's just my own thoughts, not everyone treats it the same, and it doesn't mean the way I do it is right for everyone, nor do I think it is. But our own feelings about it will also dictate how we handle these situations.