I'd like to attempt a topsy turvy cake is it reasonalbe to start with 6 inch, 4 inch and a jumbo cupcake as a top tier? (Don't want to spend too much on materials for this cake)
Thank you!

Yes, I think anything is possible! Go for it and have fun!

I know anything is possible
but will it look ok?

like muffin size cupcake? I assuming.. But 4 inch as a top tier is really tiny

the jumbo muffin is 3.5" on the top and 2" on the bottom

If you are doing a topsy turvy the 4" needs to be carved down to maximum a 2". When I do topsy turvys I usually carve down by 4" eg) 12-> 8, 10-> 6, 8-4, 6-> 2.

I think it could work, but the jumbo will look the same size as the tier below it.

Why don't you try drawing it out to scale and see what you think?

Thank , I shall try drawing it!