While I agree with Rich regarding notching your 2 x 4's to increase support, you should be fine with that configuration. The weak points would be where the "shelves" meet the posts. If you don't feel like completely rebuilding, you can always add metal joint plates or angles to help support the weight. I'll take a picture of a rack I just built last year to give you an idea of how much weight 2 x 4's can support.