Downgrade from iOS 7.02 for iPhone 4
Sorry to say that, you can't downgrade your iOS from 7.0.2 to previous iOS versions. Apple won't support this. Not only for iOS 7, but also from any iOS to its previous versions. As you have mentioned that you didn't have SHSH blobs and didn't jailbroken your iPhone, it's not possible to downgrade the OS of your iPhone.

But, you can make your OS faster in your iPhone. Refer this link and do the steps mentioned there to make your phone os speed.