How can a name that depends on it being the 4th iPhone make sense 'on it's own'? You're making it contingient on it's place within a sequence. So no, it doesn't make sense 'on it's own' but only within that sequence. Otherwise it's just a random number.
It does make sense on its own because 4 is not a random number. 4 is a number which has a significance for iPhone 4. It's the devices generation.
Exactly like 3G did make sense on its own but did not make any sense when the first model was considered as part of the series. If the first model was named iPhone Edge, then iPhone 3G would make sense according to its place in the series because both devices would have been named according to the same characteristic, the network speed.
So there is no convention on the names being entirely independent. The iPhone 4 only makes sense if it is part of a sequence and is the fourth in that sequence. It is the fourth iPhone and so is called the iPhone 4. But if that is true then the iPhone 4S is the fifth iPhone and so should be called the iPhone 5. Either one or the other is wrong.
There actually is a convention on the names being entirely independent, if there's any convention at all. If you don't count the 4S, there was absolutely no dependence between any of the names so far.
The first iPhone didn't have a name.
The second iPhone had a name and was named after the network speed it supported.
The third iPhone had a name and was named after being faster than the second iPhone.
The fourth iPhone had a name and was named after being the 4th generation in the series.
So far, no two iPhones have been named after the same principle. One for network speed, one for upgrade over the previous, one for generation.
So there's absolutely no convention, none at all.
So when Apple was naming this one, they did not have to follow any kind of convention, because they didn't create any before.
So there were many possibilities for this one. They could have named it after being the faster version of 4, which they did.
But they also could have named it iPhone HSPA+ due to the network speed it supports.
They could have named it iPhone 5, because it's the 5th iPhone.
All those would have made the same amount of sense.