Microsoft has admitted that Windows phones with under 8GB of storage and 512MB of RAM will not be receiving the Windows 10 upgrade, even if they can be expanded in storage size using an SD card.
According to NDTV a number of 4GB phones will be forever stuck on Windows 8: Lumia 530, Kazam Thunder 450W, Kazam Thunder 450WL, Karbonn Titanium Wind W4, Celkon Win 400, iBall Andi4L Pulse, BLU Win JR LTE, and Yezz Billy 4.
The problem is presumably one of needing the storage space to store the new downloaded Windows 10 to, and space to install it on to. With only 4GB to play with, there's not much space when you consider that Windows 8 is already on the phones, not to mention all of the user's content and apps and so on.
As NDTV notes, even if the phones can be expanded with a MicroSD card it doesn't appear to make a difference. Presumably Microsoft doesn't want to give users the ability to remove half an operating system and break their phone in the process.
Earlier this month Microsoft announced the first batch of Windows phones that will receive the upgrade. Among the new features are a messaging app which integrates Skype, text messages and other sources, a new photo app, and support of "Universal apps" which can run on all Windows platforms, whether mobile, desktop or (at least in theory) even Xbox One.