Cajon+Acoustic Guitar+Vocals and Keyboard.

Hi.
Hoping to get some help. I'm using an iPad mini and I want to connect 4 channels:
1. Vocals
2. Acoustic guitar
3. Cajon
4. A keyboard. (midi/synth+midi)
I Have a limited budget and it seems like the my closest option is the 'Apogee Quartet' but it costs 1300$ and that's a bit too much.
I was able to find other audio interfaces for iPad but only up to 2 channels or not compatible with iPad mini.
right now I only Have the iPad, The Cajon, And the Guitar so any thing as for which keyboard or what kind of interface I should use is flexible.
I would like all kind of advice... should I use a small mixer? or sound card? if so what kind?
recommendations for a cool vocal mic for ipad? what kind of mic should i use for the cajon?
i'm trying to design something as portable as possible... for being able to play in the streets and travel around flights and all...
Thanks a lot and sorry for asking so many questions