diff --git a/04/build.zig b/04/build.zig
new file mode 100644
index 0000000..843c820
--- /dev/null
+++ b/04/build.zig
@@ -0,0 +1,14 @@
+const Builder = @import("std").build.Builder;
+
+pub fn build(b: *Builder) void {
+    const mode = b.standardReleaseOptions();
+    const exe = b.addExecutable("04", "src/main.zig");
+    exe.setBuildMode(mode);
+    exe.install();
+
+    const run_cmd = exe.run();
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+}
diff --git a/04/src/main.zig b/04/src/main.zig
new file mode 100644
index 0000000..c5310cf
--- /dev/null
+++ b/04/src/main.zig
@@ -0,0 +1,42 @@
+const std = @import("std");
+
+const first = 171309;
+const last  = 643603;
+
+// - [x] It is a six-digit number (implied)
+// - [x] The value is within the range given in your puzzle input (implied)
+// - [x] Two adjacent digits are the same (like 22 in 122345).
+// - [x] Going from left to right, the digits never decrease; they only ever increase or stay the same (like 111123 or 135679).
+
+fn tests1(num: i32) anyerror!bool {
+    var buf: [6]u8 = undefined;
+    var numStr = try std.fmt.bufPrint(&buf, "{}", num);
+
+    if (numStr[0] > numStr[1] or
+        numStr[1] > numStr[2] or
+        numStr[2] > numStr[3] or
+        numStr[3] > numStr[4] or
+        numStr[4] > numStr[5]
+    ) return false;
+
+    return
+        numStr[0] == numStr[1] or
+        numStr[1] == numStr[2] or
+        numStr[2] == numStr[3] or
+        numStr[3] == numStr[4] or
+        numStr[4] == numStr[5];
+}
+
+pub fn main() anyerror!void {
+    var count: i32 = 0;
+    var i: i32 = first;
+
+    while (i <= last) : (i+=1) {
+        if (try tests1(i)) {
+            //std.debug.warn("Match: {}\n", i);
+            count+=1;
+        }
+    }
+
+    std.debug.warn("Part 1: matches: {}\n", count);
+}