diff --git a/day4/union_test.c b/day4/union_test.c
new file mode 100644
index 0000000..90f13e0
--- /dev/null
+++ b/day4/union_test.c
@@ -0,0 +1,23 @@
+#include <stdio.h>
+#include <string.h>
+
+union Data{
+    int i;
+    float f;
+    char str[20];
+};
+
+int main(){
+    union Data data;
+
+    data.i = 10;
+    printf("data.i: %d\n", data.i);
+
+    data.f = 3.14;
+    printf("data.f: %f\n", data.f);
+
+    strcpy(data.str, "Hello");
+    printf("data.str: %s\n", data.str);
+
+    return 0;
+}