diff --git a/Exercise 3/TestLexer.c b/Exercise 3/TestLexer.c
index 552b513..353ffc0 100644
--- a/Exercise 3/TestLexer.c	
+++ b/Exercise 3/TestLexer.c	
@@ -1,3 +1,5 @@
+/* This program is not finished! */
+
 #include <stdio.h>
 #include "Lexer.h"
 #include "Lexer.c"
diff --git a/Exercise 4/Lexer.c b/Exercise 4/Lexer.c
new file mode 100644
index 0000000..ab79784
--- /dev/null
+++ b/Exercise 4/Lexer.c	
@@ -0,0 +1,11 @@
+#include "Lexer.h"
+
+void init_Lexer(Lexer_t* lexptr, char* src_in){
+    lexptr->state = _START_;
+    lexptr->source_code = src_in;
+    lexptr->current_pos = &src_in;
+}
+
+void run_Lexer(Lexer_t* lexer) {
+
+}
diff --git a/Exercise 4/Lexer.h b/Exercise 4/Lexer.h
new file mode 100644
index 0000000..004ba8f
--- /dev/null
+++ b/Exercise 4/Lexer.h	
@@ -0,0 +1,31 @@
+/* symbols */
+#define _LETTER_ 1
+#define _DIGIT_ 2
+#define _OPERATOR_ 3
+#define _PUNCTUATION_ 4
+#define _END_OF_LINE_ 5
+#define _UNKNOWN_ 10
+
+/* state */
+#define _START_ 100
+#define _GET_NEXT_CHARACTER_ 110
+#define _READ_IDENTIFIER_ 120
+#define _READ_NUM_LIT_ 130
+#define _READ_OPERATOR_ 140
+#define _READ_PUNCTUATION_ 150
+#define _STOP_ 999
+#define _ERROR_ 1000
+
+// lexer struct
+struct Lexer_s {
+        int state;
+        char* source_code;
+        char** current_pos;
+    };
+
+// typedef
+typedef struct Lexer_s Lexer_t;
+
+// methods
+void init_Lexer(Lexer_t*, char*);
+void run_Lexer(Lexer_t*);
diff --git a/Exercise 4/TestLexer.c b/Exercise 4/TestLexer.c
new file mode 100644
index 0000000..1103e9e
--- /dev/null
+++ b/Exercise 4/TestLexer.c	
@@ -0,0 +1,15 @@
+#include <stdio.h>
+#include "Lexer.h"
+
+int main() {
+    // creating a variable of type Lexer_s
+    struct Lexer_s this_Lexer;      // declaring this_Lexer, not initialization -> random values
+    
+    // initializing the lexer with proper values
+    init_Lexer(&this_Lexer, "index = 2 * count + 42;");
+    
+    // starting the lexer
+    run_Lexer(&this_Lexer);
+
+    return 0;
+}