A band 4 is a minimum HSC mark of 70. Your current level of performance in Mathematics Advanced appears to be below that level. Of course, while the mark that you received in your last task is an indicator of your level of performance, it does not solely reflect your ability and chances of achieving an HSC mark in the band 4 range. Other information, such as your current rank relative to your cohort, as well as your school rank can assist in pinpointing the direction that you are going in at this point in time more accurately, thereby assisting you in making a more informed decision.
Unlike other universities, which emphasise assumed knowledge, that is, taking a particular subject(s) is recommended (but not compulsory) for a particular degree, USyd does indeed set prerequisites for a number of its degrees, such as the minimum of a band 4 in Mathematics Advanced in your case. Based on this, if you choose to drop Mathematics Advanced (either to Mathematics Standard or completely), you will not be eligible to receive an offer to the Bachelor of Economics/Bachelor of Laws, even if you meet the ATAR requirement.
However, as carrotsss mentioned, you can still take the Massive Open Online Course (MOOC), which is another way through which you can satisfy the maths prerequisite. This is essentially an online maths course that students take, after which they sit an exam called the post-MOOC exam. Ultimately, if a student's performance in this exam is sufficiently high, they will have satisfied the maths prerequisite.
I hope this helps!