First question: are you a developer? If so you should have a feel for what needs to be done and how long it should take. If not you need to have a strong tech lead that does and will take on the responsibility to deliver.
Expectations should be communicated, planning and estimation performed, and commitments to deliver made.
What have they to show after four months? If you are using agile the result of each sprint or iteration should be visible to the product owner. If not you have to know why. If you have seen nothing in four months I would suggest you lay down the law; give them two weeks to get a minimum viable product, or at least demonstration of progress, or they are gone.
While I agree with Ben that you need trust, and that threats do not help trust, but you must ensure they understand that they must deliver. You also need to ensure that your expectations are in line with reality. But four months work by multiple developers is a substantial effort and should see significant progress.
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