I think it is a bit rash to decide you don't like the entire field based on 3 weeks (you will be at uni for 72+ weeks) of your very first four subjects (you will take 24 or more subjects). Each subject is basically self contained and may be drastically different from the others. That said, Uni is a great time to figure out what you really want and weed out those things, its a time to try and experiment in different academic areas.
If you really, really believe you'll hate it and it's not just first year nervousness, no use going down with a sinking ship, the obvious solution is to jump off.
They will only consider any subjects completed at university (i.e. taken the full way through), so, no, your four weeks will not count. We cant tell you whether it will be competitive or not ahead of time, much like enters, these things usually aren't decided in advance (unless they're internal transfers, often these have a hard number on them, especially for the less in demand degrees, i.e. not law). If you believe you can score well in your course and this will *positively* add along to your ATAR, do it. If you will be studying things you only hate, you are not likely to do too well. In this instance, i believe it will negatively impact you as part of an overall application. It really is up to you to decide these kinds of things though.
I recommend you take the double degree over all other options where practical (i would recommend anyone take a double degree where practical or useful. Just note it may be harder to get into. At monash the rule for transferring single degrees is you need something like ~60+ish (say from Ba -> Bsc). The rule for transferring into double degrees is ~70+.