1) don't do 4P. it's pretty cumbersome and kind of a cliche - interviewers won't like it.
2) the idea is that all cases should be solved with some kind of business sense, and the frameworks are just a starting point. Through all your cases, try and get away from generic frameworks as fast as you can, as interviewers will see right through them.
2) I'd say some variation of STP could work well. Think about it as "who are you speaking to, what are you saying to them, what is the action (brand awareness, go out and buy, tell someone else to), and through what channel".
3) If you're selling something, you need to know what the something is. Ensure you understand the product first. Ask the interviewer what you're selling, and what its unique features are, and what the revenue model is underpinning it (do you make money on the initial sale or some long tail of razor blades or warranty)
4) all of this costs money, so ensure you consider the relative costs of different channels. Social media is cheap but scattershot. Discounts are expensive, but encourage adoption.
Hope this helps,