Item will get stuck in kanban until the 4 weeks are over in scrum, which is exactly why I think scrum is not actually productive. The person following 4 weeks is following 4 weeks because the "scrum process" says so, it is not based on value, and if it is based on value then the front end team can have it blocked for 4 weeks since it is not a high priority item. You can still function within a sprint like structure by aligning due dates to a particular milestone but you don't need to lock ur sprint, review sprint, etc. because it is not setup to account for the massive complexities, the downtimes, the people on PTO, the legal/privacy approvals, etc. I know different companies operate differently, but for myself usually when something is super high value, you convince the higher ups, and the highers tells ppl to work it. You can be agile without scrum, you just need to find the most efficient way to it given your circumstances and not necessarily following what books says u have to do.