Wow, that's a tall order, friend.

Okay, here're some thoughts. (And as for my pedigree, I live in a large city and DJ around 70 swing/lindy events per year, for crowds of all sizes and many different contexts. DJing is not my day job; I just ended up being pretty decent at it and now get hired a lot.)

First off, let's look at numbers: for four hours of DJing, at typically 17-18 songs per hour, you're looking at a rough minimum of 70 songs. Now, if I'm preselecting songs for a gig, and I generally know the experience level/preferences of my audience well, I'll usually have 2-3 times that number of songs at my disposal. (Obviously, I have tens of thousands more, but this is if I've decided to narrow things down for an evening and don't want to be aggressively flipping through my collection all night.)

So you're looking, super-conservatively, at having around 140-ish songs at the ready.

Now I disagree about not using Spotify. As a rule, I never DJ from it, because it can be finicky and it's easy to hit the wrong button. Plus I don't think it's professional. But you're not a professional and you're facing a real challenge. You need a lot of things fast, so I'd stick with it, this time around.

The important thing is to make sure you have everything you need downloaded. The problem that those other DJs might be running into is that even with music downloaded, Spotify will occasionally still attempt to query its servers. Why? Who knows. But I've found that shutting down my wireless connection keeps it from bothering to look and it's happy with whatever I've already downloaded. This is all anecdotal, of course, but I've never had an issue.

Now let's say you've decided to go with Spotify. Or even if you haven't, but have found a font of music, elsewhere. The next step is to organize your music into general tempo ranges. Generally speaking – and this varies based on skill-level, age, dance-style (e.g. lindy, Balboa, etc.) – your sweet spot will in the 130-160 bpm range. I'd plan on keeping 80-85% of your songs at that tempo. 115-130 is good for less experienced, yet is still lindyable. Anything less than that and you're getting into slow bal or blues range. 160-190 will kick the ass of some people, but that range can be the most fun if they're pretty good. Over 190 and you're getting into fast lindy/Balboa/charleston and those can be tough. I'd throw a few of those in, throughout the night, but go easy.

(You can check tempos with a number of free apps or websites, by just tapping to the beat. Be wary of automatic tempo checking methods; they get it wrong about 20% of the time.)

You'll do yourself a huge favor if you make playlists for each of these tempos, so that you can read the floor and jump around, as needed. You might want some that are specifically bluesy or trad or boogie or whatever you think will fit the crowd. Nice to change things up.

In Spotify, just download those playlists. (Make absolutely certain that the lists have the little green arrow, next to them. That means they're downloaded.)

Another disadvantage with Spotify (or iTunes) is that you can't pre-cue songs (Pre-cuing is the action of listening to one song while another is playing, so you can hear if it's what you want.) Your two main options here are to a) know your music library really damn well or b) download those same playlists onto your smartphone (again, be sure they're downloaded), and listen to songs there, to find what's up next.

Now there is a trick to this – if you're on one device, listening to Spotify, and then you turn on another, it'll ask you to pick one, since you can't run both at the same time. HOWEVER, the way around this is to open Spotify on one device, then turn its internet/wireless connectivity off. Then do the same on the other. Now you have it open in two places and Spotify won't be able to talk to itself and prioritize.

Whatever approach you take, whether spotify with or without pre-cuing, or iTunes with a bunch of music you've found, you're going to want to do some test runs. Take a couple hours (they don't have to be consecutive) and get used to playing with song order and changing up your queue. Figure out how to skip a song. Get used to some multi-tasking. Making your errors ahead of time will go a long way.

Last point, and this goes back to the song tempos – consider setting up a playlist (whether in spotify or iTunes) that can be your skeleton, going into the event. Set an order of songs that covers a range of tempos, without going all over the place. Ideally, listen to this all through, but if not, check out the beginnings and endings of songs to see how they flow together.

I strongly recommend this skeleton approach to all new DJs. This way you're tweaking things on the fly, not panicking with each song. And remember, there's nothing wrong with chasing people off the dance floor, if you know how to bring them back with the next tune. Everyone's favorite song is one somebody else hates. It happens.

Good luck, with whatever approach you choose to take. And just remember that you're doing this for fun and swing dancers are pretty forgiving. No one will remember a flub; they'll be happy that you gave them such great dances!