diff --git a/MuchanKim/README.md b/MuchanKim/README.md
index ef134df..8b05ac8 100644
--- a/MuchanKim/README.md
+++ b/MuchanKim/README.md
@@ -5,5 +5,6 @@
 | 1차시 | 2025.03.29 |  구현  | [개미](https://www.acmicpc.net/problem/3048)|https://github.com/AlgoLeadMe/AlgoLeadMe-14/pull/3|
 | 2차시 | 2025.04.11 |  에라토스테네스의채  | [소-난다!](https://www.acmicpc.net/problem/19699)|https://github.com/AlgoLeadMe/AlgoLeadMe-14/pull/10|
 | 3차시 | 2025.04.29 |  그리디  | [알바생 강호](https://www.acmicpc.net/problem/1758)|https://github.com/AlgoLeadMe/AlgoLeadMe-14/pull/14|
+| 4차시 | 2025.05.11 |  구현  | [오픈채팅방](https://school.programmers.co.kr/learn/courses/30/lessons/42888)|https://github.com/AlgoLeadMe/AlgoLeadMe-14/pull/20|
 
 ---
diff --git "a/MuchanKim/\352\265\254\355\230\204/\354\230\244\355\224\210\354\261\204\355\214\205\353\260\251.swift" "b/MuchanKim/\352\265\254\355\230\204/\354\230\244\355\224\210\354\261\204\355\214\205\353\260\251.swift"
new file mode 100644
index 0000000..903451a
--- /dev/null
+++ "b/MuchanKim/\352\265\254\355\230\204/\354\230\244\355\224\210\354\261\204\355\214\205\353\260\251.swift"
@@ -0,0 +1,59 @@
+import Foundation
+
+typealias UserID = String
+typealias Nickname = String
+
+enum Action: String {
+    case enter = "Enter"
+    case leave = "Leave"
+    case change = "Change"
+}
+
+func createMessage(_ action: Action, _ userId: UserID, _ userDict: [UserID: Nickname]) -> String? {
+    guard let nickname = userDict[userId] else { return nil }
+    
+    switch action {
+    case .enter:
+        return "\(nickname)님이 들어왔습니다."
+    case .leave:
+        return "\(nickname)님이 나갔습니다."
+    default:
+        return nil
+    }
+}
+
+func solution(_ record:[String]) -> [String] {
+    var res: [String] = []
+    var userDict: [UserID: Nickname] = [:]
+    var actions: [(action: Action, id: UserID)] = []
+    
+    for i in 0..<record.count {
+        let div = record[i].split(separator: " ").map { String($0) }
+        let action = Action(rawValue: div[0])!
+        let userId = div[1]
+        
+        // 닉네임 업데이트 (Enter와 Change)
+        if action == .enter || action == .change {
+            userDict[userId] = div[2]
+        }
+        
+        // 메시지 생성이 필요한 액션만 기록 (Enter와 Leave)
+        if action == .enter || action == .leave {
+            actions.append((action, userId))
+        }
+    }
+    
+    for action in actions {
+        if let message = createMessage(action.action, action.id, userDict) {
+            res.append(message)
+        }
+    }
+    
+    return res
+}
+
+// let record = ["Enter uid1234 Muzi", "Enter uid4567 Prodo","Leave uid1234","Enter uid1234 Prodo","Change uid4567 Ryan"]
+
+// let res = solution(record)
+
+// print(res)