Adding status 4 description

Author: Zach Marshall

for-research/OpenEvgenTutorial.ipynb

@@ -259,6 +259,7 @@
     "- Status code 1 means \"stable particle\"; that propagation of that particle through the detector should be simulated.\n",
     "- Status code 2 means \"particle decayed\"; there will be decay products that the generator (Pythia8 in this case) created.\n",
     "- Status code 3 means \"documentation particle\"; these are entires in the event record that the authors wanted to keep to help document what happened in the event, but they should not necessarily be considered part of any kind of physical event interpretation.\n",
+    "- Status code 4 means \"beam particle\"; these are normally the incoming two protons in the event.\n",
     "\n",
     "There are _many_ other status codes used in some generators like Pythia. \n",
     "\n",