What is the biggest difference between the 4's and 3's to you?
I have a couple options here;
Keep the RE400
Keep the 3
Get the 40's
Get the MDR-7550
The 400's seem to have the most detail.
The 3's seem good all around.
The 7550's seem to have the best bass and build quality.
MDR 7550 is a great option.
You may also wanna check Dunu Dn 1000, KEF m200, and also wait for the soon to be released Sony XBA H1, H2, H3.