diff --git a/crates/application/src/users/update_profile_fields.rs b/crates/application/src/users/update_profile_fields.rs
index 6f7fb87..2edd435 100644
--- a/crates/application/src/users/update_profile_fields.rs
+++ b/crates/application/src/users/update_profile_fields.rs
@@ -1,13 +1,9 @@
-use domain::{errors::DomainError, events::DomainEvent, value_objects::UserId};
+use domain::{errors::DomainError, events::DomainEvent, models::UserProfile, value_objects::UserId};
 
 use crate::{context::AppContext, users::commands::UpdateProfileFieldsCommand};
 
 pub async fn execute(ctx: &AppContext, cmd: UpdateProfileFieldsCommand) -> Result<(), DomainError> {
-    if cmd.fields.len() > 4 {
-        return Err(DomainError::ValidationError(
-            "Maximum 4 profile fields allowed".into(),
-        ));
-    }
+    UserProfile::validate_custom_fields(&cmd.fields)?;
     let user_id = UserId::from_uuid(cmd.user_id);
     ctx.repos
         .profile_fields
diff --git a/crates/domain/src/models/tests/mod.rs b/crates/domain/src/models/tests/mod.rs
index 4d9d0b0..94e5d57 100644
--- a/crates/domain/src/models/tests/mod.rs
+++ b/crates/domain/src/models/tests/mod.rs
@@ -230,3 +230,26 @@ fn movie_is_manual_match_different_director_fails() {
     let year = crate::value_objects::ReleaseYear::new(1982).unwrap();
     assert!(!m.is_manual_match(&title, &year, Some("Denis Villeneuve")));
 }
+
+// ── UserProfile field validation ─────────────────────────────────────────────
+
+#[test]
+fn profile_fields_validates_max_count() {
+    let fields: Vec<ProfileField> = (0..5)
+        .map(|i| ProfileField { name: format!("f{i}"), value: format!("v{i}") })
+        .collect();
+    assert!(UserProfile::validate_custom_fields(&fields).is_err());
+}
+
+#[test]
+fn profile_fields_allows_four() {
+    let fields: Vec<ProfileField> = (0..4)
+        .map(|i| ProfileField { name: format!("f{i}"), value: format!("v{i}") })
+        .collect();
+    assert!(UserProfile::validate_custom_fields(&fields).is_ok());
+}
+
+#[test]
+fn profile_fields_allows_zero() {
+    assert!(UserProfile::validate_custom_fields(&[]).is_ok());
+}
diff --git a/crates/domain/src/models/user.rs b/crates/domain/src/models/user.rs
index 3cc128c..79f364d 100644
--- a/crates/domain/src/models/user.rs
+++ b/crates/domain/src/models/user.rs
@@ -32,6 +32,20 @@ pub struct UserProfile {
     pub profile_fields: Vec<ProfileField>,
 }
 
+impl UserProfile {
+    pub const MAX_CUSTOM_FIELDS: usize = 4;
+
+    pub fn validate_custom_fields(fields: &[ProfileField]) -> Result<(), crate::errors::DomainError> {
+        if fields.len() > Self::MAX_CUSTOM_FIELDS {
+            Err(crate::errors::DomainError::ValidationError(
+                "Maximum 4 profile fields allowed".into(),
+            ))
+        } else {
+            Ok(())
+        }
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct User {
     id: UserId,