If the hole you want to re-use is in plastic, then you can simply put an M3 screw in and tighten it up - M3 is larger than 4-40. If it is in metal, then it won't work. Leaving aside the maths and dimension of the two threads...
(see imperial
http://en.wikipedia.org/wiki/Unified_Thread_Standard and metric
http://en.wikipedia.org/wiki/ISO_metric_screw_thread)
...unless the tap can cut into fresh metal, the thread will be very weak at best, and non-existant at worst. In the example above, it looks to me as though almost no load is going through the screws, and the screws are in plastic, so that would be a good application of the 'M3 for 4-40' exchange. If any load is involved, don't make the swap as the threads will be weak and will fail.
Note also that because a 4-40 thread is coarser than an M3 thread, it will work better in plastic parts than an M3, given that the hole is the right size for a 4-40 thread in the place and is 'fresh'. HTH
