Don't trust HDToolBox's recommendations. It was written before hard drives larger then 4GB existed, so it won't handle them safely (in other words, you can't trust it not to create partitions larger than your Amiga can handle). So to be safe, you should manually make sure your partitions are well below the 4GB limit.
But yes, once you have installed IDEfix and created your partitions with SFS you can access the whole 20GB disk (but your boot partition still needs to be within the first 2GB).
Edit - also note that the 4GB limit is not 4GB per partition, it is 4GB in total.
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moto