I did a Bachelor's degree in HR, but from what I have heard, the Cert IV in HR is a lot more practical and useful in terms of content than a degree. However, your opportunities for advancement within the HR field wont be nearly as strong without a degree (stupid I know, but unfortunately that's just the way it is). The Cert IV will likely limit you to admin type HR roles or officer/co-ordinator roles in small organisations. If you already have a degree, then I think it is a decent choice - a graduate certificate might be a better option though, because it is a uni level qualification.
All that being said, the HR industry is definitely driven by experience and tertiary qualifications arent as important as they are in other fields - so it is possible to go far with a TAFE diploma. Nonetheless, a degree is still a much safer bet.