Within the next few weeks, with premium processing, and only after the workflow documentation is complete. Four months leaves room for a request for evidence and a response if the petition goes in early; it does not if the documentation is still being written in month three.
Write the procedure first, then file with premium processing
The critical path for a process-based L-1B is the documentation, not USCIS. If the terminology workflow already exists as a dated manual with training records, the petition can be filed within weeks; if the procedure has to be written down for the first time, that work comes first, because a petition filed with a promise of documentation to follow may prompt a request for evidence that costs more time than preparing it before filing. Once filed with premium processing, USCIS acts within its service window; that action may be an approval or a request, and the company should reserve time to answer if one issues.
Working back from a four-month client onboarding, filing in the first month preserves more time for any response and final decision, with the analyst presenting the approval notice at the port of entry as a Canadian citizen without a visa appointment. The client should be told that the onboarding date depends on the approval, and the affiliate should identify a local fallback in case it slips.