It is safe to upgrade into 4.3.3 ( not the last 4.3.4 which closed the hole)
Only problem is that you may have problem to save your shsh. Meaning with no Blob you wouldn't be able to restore back to 4.3.3
I don't think that there is much difference from 4.2.1
i have ipad 1 ios 4.2.1 jailbreak with redsnow is it safe to upgrade to the recent ios or i will lose the ability to jailbreak. i want to upgrade because i have difficulty to transfer purchases with i tunes which i found no solution to it any where