We recently posted that Windows 10 Mobile was pretty large, which seemed to preclude having the OS on 4 Gb handsets.
NokiaPowerUser however recalled that Microsoft has posted the minimum hardware requirements for Windows 10 Mobile already some time ago.
At the time we were surprised the OS would indeed support 512 Mb RAM and 4 GB storage handsets, and NPU notes that that page made Microsoft’s strategy to provide updates for these handsets clear.
Point 2.3.4 notes:
It is not yet known of this if this will allow 4 GB Windows Phones like the Lumia 530 to be upgraded, but given the need for Microsoft to support Windows 10 Mobile on the cheapest devices to have a reasonable market share it seems likely that we will definitely see new Windows 10 Mobile handsets with 4 GB storage be released in the future.