update_engine: Increase minor version to 4

This patch increases the minor version to 4. This minor version in the
update engine adds support for three operations:
- ZERO: For zeroing blocks without writing zero bytes.
- DISCARD: For discarding blocks.
- PUFFDIFF: Reduces the size of the delta payload for boards with ARC++.
- BROTLI_BSDIFF: A new form of bsdiff patch that is 10-20% smaller.

This patch might have consequences specially for the paygen tests in canary
builds. Refer to the bug for proper handling of errors initiated from this patch
if canary builds fail. Please read the bug before reverting this patch.

BUG=chromium:782448
TEST=tryjob with canary build passes.
CQ-DEPEND=CL:658297

Change-Id: I7e4d06d55b47fd22527861f823087e3c91d996e1
Reviewed-on: https://chromium-review.googlesource.com/756940


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>