diff --git a/day4/d3.cpp b/day4/d3.cpp
new file mode 100644
index 0000000..b339e1f
--- /dev/null
+++ b/day4/d3.cpp
@@ -0,0 +1,10 @@
+#include <iostream>
+
+using namespace std;
+
+int main()
+{
+    int $123 = 10;
+    cout << $123 << endl;
+    return 0;
+}
diff --git a/day4/d4.cpp b/day4/d4.cpp
new file mode 100644
index 0000000..0b1c3f1
--- /dev/null
+++ b/day4/d4.cpp
@@ -0,0 +1,13 @@
+#include <iostream>
+
+using namespace std;
+
+int main()
+{
+    int n; // 定义变量时, 默认初始化值为 0
+    // cout << "n = " << n << endl;
+
+    // '\t' 转义字符, 表示为 <Tab> 制表符, 一般情况下, 一个制表符占 4 个空格
+    cout << n << "\t data type is " << sizeof(n) << endl;
+    return 0;
+}
\ No newline at end of file