Well they've ditched that logo as their corporate logo now so I expect All4 will be rebranded sooner rather than later. ALL fits into the 4 logo quite nicely if they go down the route of the recent multi-channel rebrand.
Or maybe not
It's still the same "4" logo, and since All 4 is the hub for all things "4", I guess they want it to be more distinctive
I wonder when Channel 4 will start putting "All" in their logo...
I don't know, for me it works even without the "ALL" part, unlike ITV Hub, which was a poor attempt in being like 4