The communication and coordination overhead of four or five developers working together is non-neglible on top of that if any part had to be reworked that also would have involved overhead between multiple developers. No matter how well you spec something out, there are always the unknowns - i.e. I forgot about this piece of business logic based on a checkbox on the front end which is going to cause a boolean to be passed in the model that’s going to affect all four services and a database table. Now I need to either communicate with all of the developers, or do it myself and dig through code I am not familiar with. In this case, I just make all six changes myself, do a quick test and commit the code and update the docs.
Then all four developers would have had to coordinate with the Devops guy.
This is all getting into “Mythical Man Month” territory.
Instead of me trying to explain it, I might as well crib the explanation from Wikipedia.
Complex programming projects cannot be perfectly partitioned into discrete tasks that can be worked on without communication between the workers and without establishing a set of complex interrelationships between tasks and the workers performing them.
Four equally qualified people should be able to deliver more features in the same amount of time working independently than four people working together.