Without blobs, you cannot downgrade your 4. On the 4S and up, once you upgrade, there's no way back even with blobs
What ios version are you on? If you're currently on 6.x, you can use iFaith (windows only) to dump your blobs. Keep that in a safe location as that'll be your ticket to downgrade back to 6.x once you upgrade to 7.x
For example, if you're on 6.1.2, you'll dump 6.1.2 blobs