Evening light can give really dramatic lighting for portrait type work with the shadow angles that it can throw across the face. One thing to be aware of is that the colour temperature will be quite different from your flash. Two toned lighting can make a statement on its own, just isn't always considered the most flattering look. Flash can be gelled to compensate. Reflectors are inexpensive to buy, can be made from white fabric as well. Would any of the folks who asked you there volunteer to assist with holding them for you? Options include lightstands, but not being voice actuated it might be a bit of a handful to manage on your own for your first event. But quality usually trumps quantity.

F4 can be made to work for people shots. Will require you to try to get shots with non-distracting backgrounds and as much distance between the subject and the background as you can manage. Not sure of how high you can go in the ISO department to maintain reasonable shutter speeds to avoid motion blurring which will ultimately depend on the lighting conditions. Glad to hear that you plan to check it out soon.

Wasn't trying to sound discouraging in earlier post. Just wouldn't someone who is too eager to jump into a situation that they aren't ready/equipped for. Would suggest taking a friend with you as a model when you go to check the venue. It'll give you a chance to see how things might work out for you with the equipment you have now and will give time to plan out how to work around any challenges you might find.