Can't display content of emails on iPhone 4
When I had this problem it was due to a network issue. Make sure that you are able to surf the web in "accurate" speed. If websites will not load, restart your phone. 

If websites are loaded as fast as ever, restart your phone nevertheless (press the key in the upper right corner for a few seconds).
I would also suggest to completely close the Mail.app before (doubletab on the home button, press and hold the mail icon until it starts wobbling, click on the red X on the corner when it appears).