Depends on individual school policies. You will still qualify for an ATAR if you complete only 4, 3/4 subjects (including English) strictly speaking, but your school might not allow you to do this.
It's also not recommended because you need to pass 4 U3&4 subjects to get an ATAR, if you fail one of your 4 it's over, but if you do 5 and fail just one you're okay.
