Firstly, relax It is possible to do four law units in a semester and lots of other students do end up doing four law units. Having said that, if you are aiming for credit or higher, do expect to do a lot of work and to have some assessments due at around the same time. You just need to have good study habits, stick to your study plans and plan ahead.
It's hard to give advice on whether you should drop down to 3 units or stay with the 4 units, this is ultimately something that you will find out when you do 4 units and see how you cope with the workload. (Do you currently work as well? If yes, will your manager give you some time off before exams? Alternatively, how do you function with minimal sleep?)
Your first semester isn't bad; 208 is not conceptually difficult, 314 (if Kelly is still teaching it) is not difficult since it should allow you to focus your attention on a few specific areas of constitutional law in your assessments and Kelly's notes are brilliant and her published guide book is worth buying. 316 has a fair bit of content to get through each week, but the content isn't too abstract so it is manageable, and if Shayne is teaching it, so long as you follow her lectures and go to the tutes + some study on your part = a credit should be manageable. 459, the subject can be fluffy and *hint hint* be prepared to do work on your part, depending on the assessment structure - focus on some areas for your assessments.
After your first semester, you will have a better idea of how you cope with 4 law units - then decide whether you want to drop down to 3 in your second semester. Your second semester's subjects are harder than your first, but not impossible and in a way, getting jurisprudence and business organisations out of the way is good. (315 and 471 shouldn't be too difficult; 315 = similar to 316, 471 = content heavy with lots of cases but if Shayne is teaching it, just do your part and you will be fine).