Open4 needs to be in the gem file.

Gemfile

 source "http://rubygems.org"
 
+gem "open4"
+
 group :development do
   gem "rake"
   gem "bacon"

Gemfile.lock

@@ -16,6 +16,7 @@ GEM
       metaclass (~> 0.0.1)
     mocha-on-bacon (0.2.0)
       mocha (>= 0.9.8)
+    open4 (1.3.0)
     rake (0.9.2.2)
     rb-fsevent (0.9.0)
     thor (0.14.6)
@@ -34,6 +35,7 @@ DEPENDENCIES
   guard-shell
   kicker
   mocha-on-bacon
+  open4
   rake
   rb-fsevent
   vcr