There are 3 options that you might be able to take advantage of:-
1. Call Att or verizon (whichever you use) and they might be able to help for a fee. Usually it will be a bit on the higher side.
2. If you have a backup of your iPhone, then open iTunes and connect your iPhone. Restore your iPhone to the previous backup. This is a free option but will erase all the current data from your iPhone and would back date it to the old data (its a like a time machine, so you go back in time and loose all the current info.) You also don't get the option to go back just for texts. Essentially, every single piece of info of your iPhone will get back dated including contacts, photos etc.
3. Get a backup recovery software which recovers the text messages for you. I'd recommend Total Saver for the price and ease of use.
To recover deleted massages on an iPhone 4, you need to connect your iPhone to your computer using the USB cord. Wait for your iTunes program to open and select the "Sync" button. Wait for your iPhone information to complete updating and remove it from your computer. Open the text message application on your iPhone and the message will be presented when it was last synced.