I understand the other comments and from an entirely logical POV, I agree.
But who cares about logic when you're talking pens?
I will admit that I love my 200s. I love the marbling and the "great pen for the price" feeling I get. And the nibs are just as satisfying to use.
But - my 400s do get more attention these days and they do feel different in my hand ("warmer"? More substantial?) than the 200s. Is it worth it from a purely logical POV? Nah. Both levels are good writers, hold the same amount of ink and only have minor cosmetic differences. From an aesthetic and very subjective POV, it is worth it. My first 400 was a second-hand blue stripe for a ridiculously good price. I was happy with that for a long time though I didn't use it as often because it felt too special compared to the 200s. I only have one other 400 (would have more if I could) and that's the Honey/White Tortoise and she is just as special but I've long gotten past my "too special to use" problem.
If you think you'd prefer the 600 size and want to hold off, you'll be just happier with that. But if you just really want to see what a 400 is like and like the 200 size, then I think you'll be pleased you made the leap. You'll still admire your 200 - they're special in their own right. Especially if you can get some of the pre-'97 pens.

But to me, there is a difference and it's enough to have me interested in other 400s. Just that I've got no pen budget at the moment. Or I should say, my budget is a goose egg.