(a) On the first attempt four coins are overturned. Now, six coins are left.
In the next turn, four more are overturned. Now only two would be left. We take one more from the left over two coins and any three from the previously turned ones. Finally, the leftover coin and the three coins from the presiding step which have already been turned twice can be overturned. Thus, in four attempts, one can complete the process.