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Welcome to the Bunnings Workshop community @Malcolm4. It's wonderful that you've joined us and many thanks for the questions about building your deck.
The size of the bearer you need to use is determined by the load they are holding and the span between your posts/footers. The fewer footers you want to put in, the larger the bearer needs to be. Once again, the size of your joists is determined by the load they are carrying and the span between your bearers. It's all interconnected. To find out the appropriate spans, you can have a look at timber span tables available online.
It wouldn't be unusual to consider bearers of Treated Pine Outdoor Timber Framing 140 x 45mm and joists of Treated Pine Outdoor Timber Framing 90 x 45mm. If you run your bearer the full 6m length, you would need around five footers, or every 1410mm. There would then be four bearers across the 4m width spaced around 1252mm apart. 15 joists would then be placed on the bearers spaced at 423mm. As mentioned above, all this needs to be verified with appropriate timber span tables. If in doubt, I encourage you to employ the services of a qualified deck builder.
You should find this guide useful when planning out your deck: How to build a low-level deck. You might also like to read through our Top 10 most popular deck projects, where you'll see examples of what size bearers and joists our members have chosen to use.
Please let me know if you have any questions.
Mitchell
