Have you had experience in a gym at all? If so, how many years? How has your education changed what you do in the gym? Do you find yourself helping others in the gym now, or giving advice to friends/family/associates? What areas of fitness do you really have a passion for and want to focus on with your clients? Do you study face-to-face, and if so, what references are your teachers/peers offering? Are there any classes you're excelling in? And, on the contrary, what subjects do you struggle with?
These are all things employers are likely to be interested in. If they know that you're part-way through Cert IV, then they know that you're a novice, that you've had minimal (or no) experience in the industry. But you probably have some basic knowledge, you probably know how to instruct exercise and put together a basic program for low-risk, general population clients. You can probably follow the employer's instructions if s/he wants you to give sessions a certain way. If you have specialised interests (eg you're really into strength and conditioning, or perhaps you used to be obese and have worked your way down to a healthy weight and know what it's like for others in the same position) then it can help with niche markets. Truth be told, most PT's I've met don't know a lot in the broad spectrum of fitness - they just know a lot about one or two distinct fields. Looking at the website for your college, it appears as though your Cert IV course takes 1-2 semesters to complete. This is a good thing. I suspect most employers with an IQ of at least 100 would be happy to see that their potential employee is doing the hard yards to get it right, rather than quickly getting their Cert IV in 3 weeks. If you study face-to-face, you probably have experience training your classmates. If not, worst-case-scenario, you can always negotiate work experience with the employer. A lot of Cert IV Fitness courses demand about a week's worth of full-time work experience spread out throughout the semester (40 hours). In even half that time, you can learn the fitness testing protocols of the business and have a few practice sessions where the employer can give feedback to whip you into shape.
Sorry for being so long-winded and blabbery, but if you want something good to come of that job interview, take these things into account. Definitely think of answers to the questions I asked at the start, because those are the areas where you can show your usefulness without having the label of having worked X amount of time in the fitness industry. I hope this helps.