Conditional on the repair work being successful, it is still expected that the work will be completed and the road reopened within the next few days, the Highways Agency said.
The motorway was shut between junctions 2 and 3 in west London last Friday when cracks were found in the viaduct structure. Initial indications were that work would require a week, but it is taking longer than expected.
Roads minister Mike Penning said: "I share the frustration of motorists at the continued delay to the opening of the M4. However, let's be absolutely clear, safety must be paramount and I will not allow the road to be reopened until this repair work has been completed successfully. This is a very complex operation and it is not possible to predict precisely how long it will take.
Cracks in the viaduct were first discovered during a planned routine investigation of the structure in March and repair work has been ongoing. Entering the final stages of the repair work, a new crack was found in a highly stressed part of the bridge supports. This necessitated a unique repair solution which involves manufacturing bespoke, specialist steel plates to install over the crack.