I would recommend spending the money on some nice glass, lights and audio and maybe a second camera for a different angle to cut to since DSLRs can only record for a max of 29:59 (some record less) because of legal reasons. I would definitely recommend getting some kind of audio gear other than the built in mic on the camera. If you have the budget I would go with 3 wireless lavs (bride, groom, official), a shotgun mic and a recorder but at least get a shotgun and a recorder. Good audio is very important especially for wedding videography. People are much more forgiving of bad video than they are of bad audio.
You really don't need 4k for a couple of reasons. People who are going to be hiring you aren't going to be asking for 4k and it's going to be a lot more expensive for you in the long run. You'll need a lot of storage space for those 4k files considering your're going to have hours of footage for just one wedding. And if you go with the BMPC and shoot 4k raw then thats at least 10x more storage space that you'll need. Also you'll need a beast of a computer to edit the 4k raw let alone 4k. As for getting DaVinci for free when you get the BMPC, DaVinci resolve is free if you're working with 1080p footage.
In the end it's your decision but if you already have a decent DSLR then there are other things you should put your money towards before upgrading to a better camera.