Getting into 4K - the Sony FDR-AX100

I just got the Sony FDR-AX100, which I think can be accurately described a being their "most inexpensive (currently) available" 4K camera. I thought it was time to at least dabble in 4K, but I didn't really have the budget to buy anything really good - no Sony F5 or Canon C500 in my budget!
But if you're not looking to spend more than $4k on 4K, there are some other choices. Why didn't I get one of those?