Did you use the stock speaker enclosures or are they just 'Free Air' mounted?
They are free air mounted. Let's be honest, these are tweeters with 4 inches of rubber for aesthetics. Adding a baffle makes a unnatural sound in my opinion, as did the original stock speaker enclosure.
Are they louder due to the 4 ohm?
I would say yes a bit, maybe, nothing drastic.
Could this harm the head unit at high power levels?
Doubtful. Everyone else on here seems to be fine running them.
Other factors, the non EV spark calls for 4 ohm speakers, most likely uses the same amp. The backs are 4 ohm and producing a amp that handles 8 ohms on 2 channels and 4 ohms on the other 2 would be very odd engineering. Why would you do that? Design a new amp for the EV just to make it have 8 ohm front speakers instead of 4 ohm?
None of these scenarios makes (especially cheap) engineering sense to me. Really we need a service manual or service tech to answer these questions, but in absent of those, I'm going to assume that the GM engineers have some common sense when designing.