For the 24" M4 iMacs, Apple shows the available storage options once on the Technical Support page – and again on the actual ordering page(s). The options vary depending on which 'stock' model you buy – or use as a starting point for custom-ordering.
Note that while you can add external storage to the M4 iMac, there is no way to expand either RAM or the internal SSD after ordering. I would suggest getting at least 512 GB of internal storage even if you intend to keep much of your bulk data on an external SSD.
