The Australian Qualification Framework provides guidelines for the learning outcomes for each AQF level and qualification type. Diplomas sit at an AQF level 5 and are therefore a higher qualification than a Certificate IV.
If you are looking to complete a Diploma rather than a Certificate IV, bear in mind that the current TAE50116 Diploma of Vocational Education and Training and TAE50216 Diploma of Trainig, Design and Development do require quite extensive evidence related to designing, delivering and assessing formal Qualifications and leading other trainer and assessor in best practice so holding the TAE40116 Certificate IV TAE and working for an RTO will ensure that you are able to complete all of the assessment requirements for these Diplomas