Extend stress test to format_version 4 (facebook#4265)

Summary:
Stress tests currently cover format_version 2 and 3. The patch adds 4 as well.
Pull Request resolved: facebook#4265

Differential Revision: D9323185

Pulled By: maysamyabandeh

fbshipit-source-id: 54d11e41ecae09bae14cadd7313f07c9a3db5a57

tools/db_crashtest.py

@@ -54,7 +54,7 @@
     "verify_checksum": 1,
     "write_buffer_size": 4 * 1024 * 1024,
     "writepercent": 35,
-    "format_version": lambda: random.randint(2, 3),
+    "format_version": lambda: random.randint(2, 4),
 }
 
 _TEST_DIR_ENV_VAR = 'TEST_TMPDIR'