Regarding those 4GB phones, it is not entirely clear that remains the final position. Many users have managed to get Windows 10 Mobile on those phones and, Microsoft using the external SD card may be the way to go forward. However, for now, the initial priority remains for those devices with 8GB or more storage.
In conclusion, do not get fixated on official updates. Although they are certainly welcomed, if you are reading this article you are likely a candidate for the Insider program. Granted, Windows 10 Mobile is still rough for daily use but this is only going to improve in the next two months. If you have any of the phones not on the official list, and you have 8GB of storage or more, you can still get Windows 10 Mobile. Indeed, you are going to not only get the upgrade but likely get it before non-Insiders.