diff --git a/Program.cs b/Program.cs
index cb26d05..d6790d1 100644
--- a/Program.cs
+++ b/Program.cs
@@ -60,7 +60,7 @@ class Program {
                     break;
                 case 4:
                     Console.Clear();
-
+                    Input4();
                     Pausa();
                     break;
                 case 5:
@@ -77,6 +77,7 @@ class Program {
         while (scelta != 0);
     }
 
+
     static void Pausa() {
         Console.WriteLine("Premere un tasto per continuare. . .");
         Console.ReadKey();
@@ -173,10 +174,16 @@ class Program {
         Console.Write("Inserire la frase da cercare: ");
         stringa = Console.ReadLine();
 
-        //converto in caratteri minuscoli per semplificare la ricerca
-        occorrenze = CercaStringa(daEsserCercato.ToLower(), stringa.ToLower());
+        if (daEsserCercato.Length < stringa.Length) {
+            Console.WriteLine("Errore: non è possibile cercare una frase più lunga in una più corta.");
+        }
+        
+        else {
+            //converto in caratteri minuscoli per semplificare la ricerca
+            occorrenze = CercaStringa(daEsserCercato.ToLower(), stringa.ToLower());
 
-        Console.WriteLine($"La stringa {daEsserCercato} contiene {stringa} {occorrenze} volte");
+            Console.WriteLine($"La stringa {daEsserCercato} contiene {stringa} {occorrenze} volte");
+        }
     }
 
     static int CercaStringa(string p_stringa1, string p_stringa2) {
@@ -196,4 +203,31 @@ class Program {
         }
         return ritorno;
     }
+
+    static void Input4() {
+        char trova, sostituisci;
+        string daEsserCercato, risultato;
+
+        Console.Write("Inserire il testo in cui cercare: ");
+        daEsserCercato = Console.ReadLine();
+        Console.Write("Inserire il carattere da cercare: ");
+        trova = Convert.ToChar(Console.ReadLine());
+        Console.Write("Inserire il carattere da sostituire: ");
+        sostituisci = Convert.ToChar(Console.ReadLine());
+
+        risultato = Sostituisci(daEsserCercato, trova, sostituisci);
+
+        Console.WriteLine($"Testo dopo la sostituzione: {risultato}");
+    }
+    static string Sostituisci(string p_stringa1, char p_lettera1, char p_lettera2) {
+        char[] lettere = p_stringa1.ToCharArray();
+
+        for (int j = 0; j < lettere.Length; j++) {
+            if (lettere[j] == p_lettera1) {
+                lettere[j] = p_lettera2;
+            }
+        }
+
+        return string.Join("", lettere);
+    }
 }
diff --git a/bin/Debug/net9.0/strings_2.dll b/bin/Debug/net9.0/strings_2.dll
index 7808df7..a268329 100644
Binary files a/bin/Debug/net9.0/strings_2.dll and b/bin/Debug/net9.0/strings_2.dll differ
diff --git a/bin/Debug/net9.0/strings_2.pdb b/bin/Debug/net9.0/strings_2.pdb
index beb55bc..9a5ed6a 100644
Binary files a/bin/Debug/net9.0/strings_2.pdb and b/bin/Debug/net9.0/strings_2.pdb differ
diff --git a/obj/Debug/net9.0/ref/strings_2.dll b/obj/Debug/net9.0/ref/strings_2.dll
index d2497a6..56639ce 100644
Binary files a/obj/Debug/net9.0/ref/strings_2.dll and b/obj/Debug/net9.0/ref/strings_2.dll differ
diff --git a/obj/Debug/net9.0/refint/strings_2.dll b/obj/Debug/net9.0/refint/strings_2.dll
index d2497a6..56639ce 100644
Binary files a/obj/Debug/net9.0/refint/strings_2.dll and b/obj/Debug/net9.0/refint/strings_2.dll differ
diff --git a/obj/Debug/net9.0/strings_2.AssemblyInfo.cs b/obj/Debug/net9.0/strings_2.AssemblyInfo.cs
index 99f455f..bc70e92 100644
--- a/obj/Debug/net9.0/strings_2.AssemblyInfo.cs
+++ b/obj/Debug/net9.0/strings_2.AssemblyInfo.cs
@@ -13,7 +13,7 @@ using System.Reflection;
 [assembly: System.Reflection.AssemblyCompanyAttribute("strings_2")]
 [assembly: System.Reflection.AssemblyConfigurationAttribute("Debug")]
 [assembly: System.Reflection.AssemblyFileVersionAttribute("1.0.0.0")]
-[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+4b05cdf4d4160be986363fb78f8cbf21b211f37c")]
+[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+4c40b3887892e33b97b5bc6143d5052b0d5ff603")]
 [assembly: System.Reflection.AssemblyProductAttribute("strings_2")]
 [assembly: System.Reflection.AssemblyTitleAttribute("strings_2")]
 [assembly: System.Reflection.AssemblyVersionAttribute("1.0.0.0")]
diff --git a/obj/Debug/net9.0/strings_2.AssemblyInfoInputs.cache b/obj/Debug/net9.0/strings_2.AssemblyInfoInputs.cache
index 13ab16f..0d68760 100644
--- a/obj/Debug/net9.0/strings_2.AssemblyInfoInputs.cache
+++ b/obj/Debug/net9.0/strings_2.AssemblyInfoInputs.cache
@@ -1 +1 @@
-299f8e2b0aa8572acb3ff58479c8e5c281de4a1c0f7fd9ae96c9953faf5173fc
+587bb6324f88ac6117ac0918a7ee338a0587fa64ef322f6a201f5a253d017309
diff --git a/obj/Debug/net9.0/strings_2.dll b/obj/Debug/net9.0/strings_2.dll
index 7808df7..a268329 100644
Binary files a/obj/Debug/net9.0/strings_2.dll and b/obj/Debug/net9.0/strings_2.dll differ
diff --git a/obj/Debug/net9.0/strings_2.pdb b/obj/Debug/net9.0/strings_2.pdb
index beb55bc..9a5ed6a 100644
Binary files a/obj/Debug/net9.0/strings_2.pdb and b/obj/Debug/net9.0/strings_2.pdb differ