In my experience, payment in arrears is generally on the grounds...

In my experience, payment in arrears is generally on the grounds of a history of well-grounded rapport or to develop rapport, or for contingent processes where costs can be estimated but only imprecisely forecast (which would still require rapport). 4DS and IMEC have a history at this point, so there's your rapport - though it is worth noting that payment in arrears benefits 4DS rather than IMEC. In my professional experience, there wouldn't be a lot of reason to read too deeply into the arrears thing. Even if it were not in arrears, IMEC would be on the hook at minimum to refund payment for bad manufacturing processes and shipping failures, and it would be an incredibly bad look on their part if they didn't live up to that commitment. Even with compensation, IMEC would be swimming hard against the current to rebuild trust with their other clients.

Conversely, if you wanted to, I suppose a question would be 'If 4DS is slated to sell, who is the rapport for?' That's an interesting thought experiment because IMEC could be in a position to continue playing a role for whoever buys it, and playing a key role at this point puts them closer to the front of the line. The not so wonderful alternative is that the rapport is for 4DS because IMEC is gambling that this iteration won't be the winner and 4DS are going to need some more IMEC service in the future.

At any rate, this all strikes me as trying to read the tea leaves when you drank bagged tea.