diff --git a/bidi.lua b/bidi.lua
index 1c069ce..d98eb9b 100644
--- a/bidi.lua
+++ b/bidi.lua
@@ -230,7 +230,7 @@ assert(
    "R" == test_output[3].bidi_class
 )
 
-bidi.W4 = function(run)
+local W4 = function(run)
    for i = 1, #run do
       if "ES" == run[i].bidi_class then
          -- A single European separator between two European numbers
@@ -268,6 +268,45 @@ bidi.W4 = function(run)
    return run
 end
 
+-- Test W4: EN ES EN → EN EN EN
+local test_input = {
+   { ["bidi_class"] = "EN" },
+   { ["bidi_class"] = "ES" },
+   { ["bidi_class"] = "EN" },
+}
+local test_output = W4(test_input)
+assert(
+   "EN" == test_output[1].bidi_class and
+   "EN" == test_output[2].bidi_class and
+   "EN" == test_output[3].bidi_class
+)
+
+-- Test W4: EN CS EN → EN EN EN
+local test_input = {
+   { ["bidi_class"] = "EN" },
+   { ["bidi_class"] = "CS" },
+   { ["bidi_class"] = "EN" },
+}
+local test_output = W4(test_input)
+assert(
+   "EN" == test_output[1].bidi_class and
+   "EN" == test_output[2].bidi_class and
+   "EN" == test_output[3].bidi_class
+)
+
+-- Test W4: AN CS AN → AN AN AN
+local test_input = {
+   { ["bidi_class"] = "AN" },
+   { ["bidi_class"] = "CS" },
+   { ["bidi_class"] = "AN" },
+}
+local test_output = W4(test_input)
+assert(
+   "AN" == test_output[1].bidi_class and
+   "AN" == test_output[2].bidi_class and
+   "AN" == test_output[3].bidi_class
+)
+
 bidi.W5 = function(run)
    for i = 1, #run do
       if "ET" == run[i].bidi_class then
@@ -337,7 +376,7 @@ bidi.resolve_weak_types = function(run, sos)
    run = bidi.W1(run, sos)
    run = W2(run, sos)
    run = W3(run)
-   run = bidi.W4(run)
+   run = W4(run)
    run = bidi.W5(run)
    run = bidi.W6(run)
    run = bidi.W7(run, sos)