Likely to be very little queue - so you could give it a go 1st and see what you think.
I'm not sure I'd describe as 'dark' - more dim lighting. Obviously not so 'cartoony' and colourful as Space Ranger Spin- but monsters aren't overly scary, I can't remember anything 'jumping out' which would be scary for a 4yr old. Your vehicles go though 'evening' street scenes and you shoot monsters. At one point other vehicles can 'shoot' your vehicle which makes it spin suddenly - I'd warn her about that. The cars hold 5-6 people so families can all go together.
MIB didn't ever frighten my DD but she was slightly older 6-7; and she was afraid of the dark. There are still rides/shows she won't do. Tough to be a bug in AK for example.