4. There are 49 visually identical coins, among which 25 are genuine and 24 are counterfeit. There is a device available to identify the authenticity of the coins; you can place any number of coins into the device, and if the number of counterfeit coins exceeds half, the device will signal. How can you use this device to measure 5 times and find at least two counterfeit coins?
4. If two counterfeit coins can be found from a pile of coins, then this pile of coins is called an "operational object".

For a pile containing no less than $M$ counterfeit coins and a total of $N$ coins, it is denoted as $N: M$.
Obviously, the initial operational object is 49: 24.
Next, for each operation, “$(+)$” indicates that the next operational object is the pile of all coins put into the instrument this time; “$(-)$” indicates that the next operational object is the pile of all coins not put into the instrument this time.

First operation: Put 27 coins into the instrument. There are $(+) 27: 14 ;(-) 22: 11$, which means if there are no less than 14 counterfeit coins among the 27 coins put into the instrument, then these 27 coins will be the next operational object; otherwise, there are no less than 11 counterfeit coins among the remaining 22 coins, and these 22 coins will be the next operational object.

Second operation: When the operational object is $27: 14$, then put 16 coins into the instrument. There are $(+) 16: 9$; ( - )11:6. When the operational object is 22: 11, then put 11 coins into the instrument. Regardless of $(+)$ or $(-)$, there is 11:6.

Third operation: (1) When the operational object is $16: 9$, then put 8 coins into the instrument. Regardless of $(+)$ or $(-)$, there is $8: 5$. Thus, the fourth operation will put 4 coins into the instrument. Regardless of $(+)$ or $(-)$, there is $4: 3$. The fifth operation will put two coins into the instrument. Regardless of $(+)$ or $(-)$, there is $2: 2$, meaning two counterfeit coins have been found.
(2) When the operational object is $11: 6$, then put 8 coins into the instrument. There are $(+) 8: 5 ;(-) 3: 2$. If it is $8: 5$, then proceed with the fourth and fifth operations as mentioned earlier; if it is $3: 2$, you can put one coin into the instrument twice. If the instrument signals twice, it indicates that both are counterfeit coins; if it signals only once, it indicates that the coin put in during that signal and the remaining coin are both counterfeit.