Of course CP (not just FTJ, but general CP) can be done who are already on H4. Some people go to CP path to void long wait in 485 stage.
Regarding FTJ, it can be pursued for a spouse who is already in USA, but that does not make sense. It's normally pursued for them who are unable live in USA for long period of time due to various reasons - job, family issue, study or even immigration issue. For example, in case of "vinsus" (the creator of this thread), her spouse cannot come to USA for years. Normally, AP should cover that issue (assuming visa is already expired). But AP is valid for only one year (and it should be obtained while living in USA). If spouse can come to USA once in a year, apply for new AP and go back again after he/she receives that, that way spouse can keep 485 alive. But for some spouse, that might not be a possibility - because AP takes more than a month to get approved.
On the other hand, keeping visa valid (like H1/H4) another way to keep 485 alive because there is no one year restriction.