paycheck: Support minor version 4.

From https://chromium-review.googlesource.com/#/c/333160/

Bug: 27156473
Test: ./scripts/update_payload/checker_unittest.py

Change-Id: I940debd5c878f622970e214fce75588f96d78407
diff --git a/scripts/update_payload/common.py b/scripts/update_payload/common.py
index 88cd6ed..678fc5d 100644
--- a/scripts/update_payload/common.py
+++ b/scripts/update_payload/common.py
@@ -27,6 +27,7 @@
 INPLACE_MINOR_PAYLOAD_VERSION = 1
 SOURCE_MINOR_PAYLOAD_VERSION = 2
 OPSRCHASH_MINOR_PAYLOAD_VERSION = 3
+IMGDIFF_MINOR_PAYLOAD_VERSION = 4
 
 #
 # Payload operation types.