Can IELTS 4.5 go to Australia for postgraduate preparation now, or try to take language courses first and apply for 570 or 575?

No, IELTS requires at least 5 points. 570 is a middle school visa. Only middle school students can apply, and it is difficult to approve the 570 visa now, especially in Fujian (I don't know where you are from). If you want to study in Australia, it is strongly recommended that you study in Western Australia or South Australia. East Australia is too difficult to mix, especially Sydney. It seems that it is difficult to find a job, but if you don't need a job, it is another matter. The higher the IELTS score, the better, whether you want to emigrate or be a turtle.