I am afraid you might have to do a restore, and then, JB again. But, to restore to 4.2.1, you need to have saved 4.2.1 SHSH blobs. W/o it, you can only restore to 4.3. And, if you haven't previous saved your 4.2.1 blobs, you won't be able to do it now. To be sure, download and run Tinyumbrella. Make sure your iPhone is detected. Click on "Save my SHSH...". Afterward, what SHSH blobs were you able to saved? Whatever you were able to saved are what you can restore to.