diff --git a/changes.d/6248.fix.md b/changes.d/6248.fix.md
new file mode 100644
index 00000000000..88e5d0ab279
--- /dev/null
+++ b/changes.d/6248.fix.md
@@ -0,0 +1 @@
+Upgrade to EmPy 4 to fix an issue with stdout in detaching mode.
diff --git a/conda-environment.yml b/conda-environment.yml
index 4a598468d67..de911f87866 100644
--- a/conda-environment.yml
+++ b/conda-environment.yml
@@ -21,7 +21,7 @@ dependencies:
   - tomli >=2 # [py<3.11]
 
 # optional dependencies
-  #- empy >=3.3,<3.4
+  #- empy >=4.1
   #- pandas >=1.0,<2
   #- pympler
   #- matplotlib-base
diff --git a/cylc/flow/parsec/empysupport.py b/cylc/flow/parsec/empysupport.py
index b4164894e0f..255cc36dc55 100644
--- a/cylc/flow/parsec/empysupport.py
+++ b/cylc/flow/parsec/empysupport.py
@@ -52,7 +52,11 @@ def empyprocess(
     os.chdir(dir_)
     ftempl = StringIO('\n'.join(flines))
     xtempl = StringIO()
-    interpreter = em.Interpreter(output=em.UncloseableFile(xtempl))
+    # Requires EmPy >= 4.1
+    interpreter = em.Interpreter(
+        output=em.UncloseableFile(xtempl),
+        dispatcher=False  # raise errors to caller
+    )
 
     # Add `CYLC_` environment variables to the global namespace.
     interpreter.updateGlobals(
@@ -60,7 +64,7 @@ def empyprocess(
     )
 
     try:
-        interpreter.file(ftempl, '<template>', template_vars)
+        interpreter.file(ftempl, locals=template_vars)
     except Exception as exc:
         lineno = interpreter.contexts[-1].identify()[1]
         raise EmPyError(
diff --git a/setup.cfg b/setup.cfg
index be1e9383431..52d97428748 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -87,7 +87,7 @@ include = cylc*
 
 [options.extras_require]
 empy =
-    EmPy==3.3.*
+    EmPy>=4.1
 graph =
     pillow
 main_loop-log_data_store =