diff --git a/caddyconfig/httpcaddyfile/addresses.go b/caddyconfig/httpcaddyfile/addresses.go
index 710532047bd..a4f07ae60db 100644
--- a/caddyconfig/httpcaddyfile/addresses.go
+++ b/caddyconfig/httpcaddyfile/addresses.go
@@ -337,7 +337,9 @@ func (a Address) Normalize() Address {
 	// ensure host is normalized if it's an IP address
 	host := strings.TrimSpace(a.Host)
 	if ip := net.ParseIP(host); ip != nil {
-		host = ip.String()
+		if ipv6 := ip.To16(); ipv6 != nil && ipv6.DefaultMask() == nil {
+			host = ipv6.String()
+		}
 	}
 
 	return Address{
@@ -349,28 +351,6 @@ func (a Address) Normalize() Address {
 	}
 }
 
-// Key returns a string form of a, much like String() does, but this
-// method doesn't add anything default that wasn't in the original.
-func (a Address) Key() string {
-	res := ""
-	if a.Scheme != "" {
-		res += a.Scheme + "://"
-	}
-	if a.Host != "" {
-		res += a.Host
-	}
-	// insert port only if the original has its own explicit port
-	if a.Port != "" &&
-		len(a.Original) >= len(res) &&
-		strings.HasPrefix(a.Original[len(res):], ":"+a.Port) {
-		res += ":" + a.Port
-	}
-	if a.Path != "" {
-		res += a.Path
-	}
-	return res
-}
-
 // lowerExceptPlaceholders lowercases s except within
 // placeholders (substrings in non-escaped '{ }' spans).
 // See https://github.com/caddyserver/caddy/issues/3264
diff --git a/caddyconfig/httpcaddyfile/addresses_test.go b/caddyconfig/httpcaddyfile/addresses_test.go
index 612ef67ef5a..232460d0ffb 100644
--- a/caddyconfig/httpcaddyfile/addresses_test.go
+++ b/caddyconfig/httpcaddyfile/addresses_test.go
@@ -106,67 +106,128 @@ func TestAddressString(t *testing.T) {
 func TestKeyNormalization(t *testing.T) {
 	testCases := []struct {
 		input  string
-		expect string
+		expect Address
 	}{
 		{
-			input:  "example.com",
-			expect: "example.com",
+			input: "example.com",
+			expect: Address{
+				Host: "example.com",
+			},
 		},
 		{
-			input:  "http://host:1234/path",
-			expect: "http://host:1234/path",
+			input: "http://host:1234/path",
+			expect: Address{
+				Scheme: "http",
+				Host:   "host",
+				Port:   "1234",
+				Path:   "/path",
+			},
 		},
 		{
-			input:  "HTTP://A/ABCDEF",
-			expect: "http://a/ABCDEF",
+			input: "HTTP://A/ABCDEF",
+			expect: Address{
+				Scheme: "http",
+				Host:   "a",
+				Path:   "/ABCDEF",
+			},
 		},
 		{
-			input:  "A/ABCDEF",
-			expect: "a/ABCDEF",
+			input: "A/ABCDEF",
+			expect: Address{
+				Host: "a",
+				Path: "/ABCDEF",
+			},
 		},
 		{
-			input:  "A:2015/Path",
-			expect: "a:2015/Path",
+			input: "A:2015/Path",
+			expect: Address{
+				Host: "a",
+				Port: "2015",
+				Path: "/Path",
+			},
 		},
 		{
-			input:  "sub.{env.MY_DOMAIN}",
-			expect: "sub.{env.MY_DOMAIN}",
+			input: "sub.{env.MY_DOMAIN}",
+			expect: Address{
+				Host: "sub.{env.MY_DOMAIN}",
+			},
 		},
 		{
-			input:  "sub.ExAmPle",
-			expect: "sub.example",
+			input: "sub.ExAmPle",
+			expect: Address{
+				Host: "sub.example",
+			},
 		},
 		{
-			input:  "sub.\\{env.MY_DOMAIN\\}",
-			expect: "sub.\\{env.my_domain\\}",
+			input: "sub.\\{env.MY_DOMAIN\\}",
+			expect: Address{
+				Host: "sub.\\{env.my_domain\\}",
+			},
 		},
 		{
-			input:  "sub.{env.MY_DOMAIN}.com",
-			expect: "sub.{env.MY_DOMAIN}.com",
+			input: "sub.{env.MY_DOMAIN}.com",
+			expect: Address{
+				Host: "sub.{env.MY_DOMAIN}.com",
+			},
 		},
 		{
-			input:  ":80",
-			expect: ":80",
+			input: ":80",
+			expect: Address{
+				Port: "80",
+			},
 		},
 		{
-			input:  ":443",
-			expect: ":443",
+			input: ":443",
+			expect: Address{
+				Port: "443",
+			},
 		},
 		{
-			input:  ":1234",
-			expect: ":1234",
+			input: ":1234",
+			expect: Address{
+				Port: "1234",
+			},
 		},
 		{
 			input:  "",
-			expect: "",
+			expect: Address{},
 		},
 		{
 			input:  ":",
-			expect: "",
+			expect: Address{},
 		},
 		{
-			input:  "[::]",
-			expect: "::",
+			input: "[::]",
+			expect: Address{
+				Host: "::",
+			},
+		},
+		{
+			input: "127.0.0.1",
+			expect: Address{
+				Host: "127.0.0.1",
+			},
+		},
+		{
+			input: "[2001:db8:85a3:8d3:1319:8a2e:370:7348]:1234",
+			expect: Address{
+				Host: "2001:db8:85a3:8d3:1319:8a2e:370:7348",
+				Port: "1234",
+			},
+		},
+		{
+			// IPv4 address in IPv6 form (#4381)
+			input: "[::ffff:cff4:e77d]:1234",
+			expect: Address{
+				Host: "::ffff:cff4:e77d",
+				Port: "1234",
+			},
+		},
+		{
+			input: "::ffff:cff4:e77d",
+			expect: Address{
+				Host: "::ffff:cff4:e77d",
+			},
 		},
 	}
 	for i, tc := range testCases {
@@ -175,9 +236,18 @@ func TestKeyNormalization(t *testing.T) {
 			t.Errorf("Test %d: Parsing address '%s': %v", i, tc.input, err)
 			continue
 		}
-		if actual := addr.Normalize().Key(); actual != tc.expect {
-			t.Errorf("Test %d: Input '%s': Expected '%s' but got '%s'", i, tc.input, tc.expect, actual)
+		actual := addr.Normalize()
+		if actual.Scheme != tc.expect.Scheme {
+			t.Errorf("Test %d: Input '%s': Expected Scheme='%s' but got Scheme='%s'", i, tc.input, tc.expect.Scheme, actual.Scheme)
+		}
+		if actual.Host != tc.expect.Host {
+			t.Errorf("Test %d: Input '%s': Expected Host='%s' but got Host='%s'", i, tc.input, tc.expect.Host, actual.Host)
+		}
+		if actual.Port != tc.expect.Port {
+			t.Errorf("Test %d: Input '%s': Expected Port='%s' but got Port='%s'", i, tc.input, tc.expect.Port, actual.Port)
+		}
+		if actual.Path != tc.expect.Path {
+			t.Errorf("Test %d: Input '%s': Expected Path='%s' but got Path='%s'", i, tc.input, tc.expect.Path, actual.Path)
 		}
-
 	}
 }