Hm. I have never had a problem with iPhone 4's on 4.2.1 jb with GP. I had 2 of those in my household before switching to VZW. It might just be the app you're using too.
Well I'm not even gonna worry about at this point! lol
I was looking into tiny umbrella just now when I realized that it said for her phone there was a 4.2.1 shsh saved! How is that possible when I know for a fact that she never upgraded past 4.1?
You don't have to be on a certain firmware to have SHSHs for it. You can be on 3.1.3, save your SHSHs before Apple stops signing a particular firmware, and have 4.0 all the way up to 4.2.1 saved without having to upgrade to it. Cydia automatically saves any new SHSHs available once you authorize it to save the very first time.