So I spent the good part of 2 hours setting up my 4 line kite, I triple checked it and everything was perfect...
I went to go and fly the kite, it refused to fly.
So I attached my handles to a pole and chucked my kite up when there was a lot of wind, it went straight down... It was the correct way up (Holes at the top).
I'm not sure what went wrong, do you know what it may have been?

10-16 MPH wind.

I saw in your other thread that you were concerned about long brake lines. Did you shorten them? If so, are your brake lines too short now?

Yes, I shortened them following the advice of:
"Just shorten the lines by tying a loop in the brake lines approximately where they need to be with a double overhand knot, cut off the excess, then do the fine tuning by tying knots in the break leaders on the bottom of the handle and moving the break lines in and out to fine tune."

That worked like a charm, although I didn't cut the excess line just to be safe (It was the size of a 2 pence coin, in a 3D aspect).

After there was maybe 0.75 meters difference between the power and brake lines.

See if the kite will fly without the brake lines. If it will without them and then won't with the brake lines attached, your brake lines are too short and are in a constant "on" configuration. . In general, the brake lines will not be very tight unless you are applying brakes. There are of course exceptions and some kites have more bias towards brake input but your circumstances that you've relayed here all indicate that you have the brake lines too short. try letting the brakelines out about 15-20cm and test again.

Like the others have said , you sound like you over shortened the brakes.

Well built kites are set up for all 4 lines to be equal in length. Your budget kite may not be set up that way.

It should fly as a 2 line . Try it that way. If it does, add a bunch of length back to your brake lines. It should fly on the front lines again. Now start trimming back on the brake line so they have a bit of slack ( an arc ) with the bottom of your handle relaxed / pointing toward the kite. You will have it trimmed right when you can go from front line tension to being able to back the kite down with rear line tension . Pulling the brake lines / bottom of handles in.

Not sure what exactly is going on, where are you located? If someone is not close enough to you to help you out, if you are in the US, ship it to me, I will tune it for you the best I can and send it back to you.

kbkasey dont forget to point the bottom of the handles toward the kite so as to allow "a little" slack in the brake lines. If those brake lines are tight from being too short or from you tipping the handles back on the bottom you will have trouble flying it.

start with the brakes completely slack and see if it flies. If it does, than start shortening the brake lines to the point where it will give you added control. You should have it set up correctly when you are flying mainly off the top 2 lines and when you apply the brakes the kite will land / stall or at least try to.

Snowspider is right. it is easy for new flyers to unintentionally apply brakes when flying.