i believe iphone 4s and 5 have welded glass or something making it much harder to replace alone. as a result usually they replace lcd+glass which make it much more expensive.
You can generally buy the digitizer + lcd on ebay for about 30 bucks. It's a lot of small screws, but really not too difficult to swap out. Took me about 45 minutes.
For any phone I would recommend buying the digitizer/lcd combined. It's such a pain to separate the two (on phones where it's even possible) and only slightly more expensive.