Acts 7:6

‘And God spoke in this vein, that his seed would sojourn in a strange land, and that they would bring them into bondage, and treat them ill, four hundred years.'

Nor did God promise immediate possession of the land for his seed. They also would be away from the land for four hundred years (Genesis 15:13). Thus it was clearly not their possession of the land that mattered, but that they were His people, with a future hope. They would indeed live in a strange land. And there they would in time be in bondage, and would be ill-treated (as Stephen and his hearers were being in Palestine at that time under Roman rule). The ‘four hundred years' relates to ‘sojourn', not to the being in bondage, which would be for only part of that time. But both would be with a future hope.