1) Do you actually need GPS and maps in your daily life, or is this a case of hedonic adaptation and/or an excuse to not plan ahead?
2) How often do you find yourself going, "Gee whiz, I really need to take a picture of this?" let alone, "Crap, I wish the camera on this device wasn't so terrible for the pictures I've needed to take!"
3) You said "pseudo-blackberry" and not a real Blackberry, which means there's no reliance on BIS which is the only hurdle to using a Blackberry on an MVNO (beyond the normal hurdles for any handset). What exactly is keeping you from taking your current phone to an MVNO in the first place?
4) Do you actually need the ability to access All Knowing Lord Google everywhere you go, or is this yet again a case of hedonic adaptation and/or a supposed cure for boredom and/or social peer pressure (AKA keeping up with the Joneses)?
As to the iPhone 4 itself, iOS7 has proven to be quite the turkey so far, and is bloated enough that it's a bit of a dog on the hardware. This isn't to say, however, that you have to upgrade the firmware from iOS6 especially since Apple is allowing for access to older versions of software for older OS versions now. Also, a replacement screen can be had for under $30 off Ebay. A decent replacement battery will probably cost you between $15-20 off Ebay as well and you'll likely need about $10-15 worth of tools to do all the repairs and you can potentially break the phone worse if you're not careful and don't take your time. I also brought up the battery because if it's over two years old, the battery life on that sucker's most likely only getting rougher with each power cycle. Also, you're buying a piece of electronics that had an abusive lifespan given the broken screen. On the flip side to all this, I've seen 8GB iPhone 4 models go for under $200 refurbished if you're patient. I've also seen refurbished Android handsets that could easily do everything the iPhone 4 supposedly can for the most part for half that and you don't need custom tools to replace the danged battery or deal with the charlie foxtrot of potentially needing to run iTunes under Windows.
From an MVNO and data usage standpoint, the iPhone uses more background data than other smartphones and the OS is designed for pretty much an all or nothing setup with data access, so you either need to completely lose the convenience of always on data or be prepared to use a lot more data than you probably actually need if you want the convenience of push email or text replacement apps. This isn't to say that you can't just turn off 3G data access except when you need it, but if you're looking at it from a usable tool standpoint, it's something else to consider.