I am doing a wedding cake for a friend of mine...the wedding is in two weeks. She had a lot of different ideas for it, but we have finally have it figured out.

The cake is going to be 4 tiers: 6in,8in, 10in, and 12in. Now she has decided that she wants 'spacers' between each tier. So, I ordered 7 dummies: 3 for the spacers and 4 the same size as the cakes. (I thought it would be nice to play with). I wasn't sure what the size difference should be, so I ordered the 6, 8, 10, and 12 in dummies 4in thick, and the others are 5, 7, and 9 in 3in thick.

I got them in today...but it looks funny to me. The bottom tiers look fine, but its the top two that look a little off... I'm wondering if the spacers should have been 2 in high instead of 3...

What would you recommend?

The size of the spacers to use is just what looks best to you. Personally I prefere 2" tall spacers. I also prefere smaller ones. I'd probably would have gone with 4, 6, and 6" spacers between the tiers. But that's just the look I like.
The smaller the diamater, the taller the round will look. That's why you think the two top ones look off. The eye is fooled to think it is taller than it is.
When I make a cake w/a 6" round toptier I almost always make it 3" tall instead of 4" = even when the otheer tiers are all 4" tall.
I think once the whole cake is put together you will find it looks good. Don't get all stressed out about it

Personally I like my spaces 2" tall and 2" round less than the tier it is supporting.

Thats the beuty of playing with dummy cakes, you can test drive a cake design. If you really dont like it and have time to order new spacers then do that and try it again- cake dummies never go to waste!