You need old saved SHSH files to downgrade to a firmware Apple won't allow. Thus, if you want to downgrade to 4.1, you must have saved 4.1 SHSH files and so on. And, what SHSH files you can save has nothing to do with what your phone is currently running. It only has do with what SHSH files Apple is currently sending out, right now only 4.2.1. Also, your phone does not need to be JB to save SHSH files. Just use Tinyumbrella.