Bump up the BBOTA version to 4.

To accommodate new changes (such as error correction in [1]) to BBOTA
in N release. We bump up the version to keep the OTA script backward
compatible.

Needs the matching CL in commit
1fdec8685af858c5ff4f45d2e3059186ab5ed2ab.

[1]: commit 0a7b47397db3648afe6f3aeb2abb175934c2cbca

Change-Id: Ib9158b455cd5905fe2d4742ce81feb1b7583054f

tools/releasetools/blockimgdiff.py

@@ -241,7 +241,7 @@ class Transfer(object):
 # original image.
 
 class BlockImageDiff(object):
-  def __init__(self, tgt, src=None, threads=None, version=3):
+  def __init__(self, tgt, src=None, threads=None, version=4):
     if threads is None:
       threads = multiprocessing.cpu_count() // 2
       if threads == 0:
@@ -252,7 +252,7 @@ class BlockImageDiff(object):
     self.src_basenames = {}
     self.src_numpatterns = {}
 
-    assert version in (1, 2, 3)
+    assert version in (1, 2, 3, 4)
 
     self.tgt = tgt
     if src is None: