Try to break down the tasks so that their single task doesn't take more than a day/two, see how it works out. Some people seem to operate better on short, simple tasks with visible understandable impact.
Also a 4 weeks task for a member of a larger team is something that turns them into their own development shop and it's probably not something you want to have in a team.
I also understand some will say "a senior dev should manage such a long task on their own" - well maybe they should and some might, but in the end it's not their business and they can jump the ship 2 weeks before the deadline of a 3 months task (seen that) - that's why good project/tech/team management is a skill ))
Also a 4 weeks task for a member of a larger team is something that turns them into their own development shop and it's probably not something you want to have in a team.
I also understand some will say "a senior dev should manage such a long task on their own" - well maybe they should and some might, but in the end it's not their business and they can jump the ship 2 weeks before the deadline of a 3 months task (seen that) - that's why good project/tech/team management is a skill ))