I’m sorry no one has replied to you over the last 15 days, but I am now going to change that.
You seem to have LINKED Assets rather than embedded, just like me. I always run Blocs 4.0.1 from the Dock and never double-click it from inside the Applications folder, so I honestly do not see how that would be any kind of workaround for unlinked or missing Assets at all.
I have, from time to time, experienced a weird situation in Blocs 4 and even in Blocs 3.5 where I open a Blocs document and then Blocs complains that a select few graphics are missing when in fact they aren’t, nor were they moved, nor were they renamed. I am then forced to take Blocs by the hand and show it where one of those graphics are, which of course is the location of the rest of the graphics it is complaining to me about, and then Blocs seems satisfied and I can go on with my work. I cannot explain why that happens. Even so, you seem to have a different case where ALL your linked graphics are missing, and that is truly a mystery to me, especially if you never saw that before in Blocs 3.x.
However, if your linked graphics are remotely hosted, then such could explain the problem. All my graphics are saved locally on my iMacs internal SSD, along with Blocs. And of course, if you moved any folders or renamed any folders, such would explain it, although it doesn’t sound like you did any of that.
So the question now becomes, after you solved the linked Assets problem and then saved your document. What happens when you relaunch Blocs 4 FROM THE DOCK and open that same document again? Is the problem gone or is it recurring? And does double-clicking from within the Applications folder really solve the problem every time? If it is a recurring problem, you need to submit a Blocs 4.x Bug Report here.