Log Message:
Redis 4.0.1

Bug fixes:
- Loading two or more modules exporting native data types resulted
  into the inability to reload the RDB file.
- Crash in modules when calling from Lua scripts module commands that
  would block.
- A Redis Cluster crash due to mis-handling of the "migrate-to"
  internal flag.
- Other smaller fixes not worth of a release per se, but nice to add
  here.

Redis 4.0.0

Major features

- Redis modules system. Redis now allows developers to write modules
  that can extend the Redis functionalities and implement new data
  types.
- Partial Replication (PSYNC) version 2.
- Cache eviction improvements. Redis 4.0 implements LFU (Least
  Frequently Used) as a new eviction algorithm, and improves the
  functionality, performances and precision of the existing algorithms.
- Lazy freeing of keys. Redis is now able to delete keys in the
  background in a different thread without blocking the server.
- Mixed RDB-AOF format. If enabled the new format is used when
  rewriting the AOF file: the rewrite uses the more compact and faster
  to generate RDB format, and an AOF stream is appended to the file.
- A new MEMORY command, able to perform memory analysis of different
  kinds: troubleshooting of memory issues (with MEMORY DOCTOR, similar
  to LATENCY DOCTOR), reporting of the amount of memory used by a single
  key, more in-depth reporting of Redis memory usage compared to what
  the INFO command offers.
- Redis Cluster support for NAT / Docker.
- Redis uses now less memory in order to store the same amount of
  data. The gain depends a lot on the kind of dataset stored.
- Redis is now able to defragment the used memory and reclaim space
  incrementally while running.

Smaller features

- Improvements to the RDB format to support 64 bit lengths, binary
  sorted set scores, and more.
- SWAPDB command: ability to completely and immediately (no latency)
  replace two Redis databases.
- Improvements to `dict.c`, the Redis hash table implementation.
- Security improvements mapping POST and Host: commands to QUIT in
  order to prevent cross protocol scripting attacks.
- RPUSHX and LPUSHX now accept a variable number of elements.
- Reporting of additional memory used by copy on write in the INFO
  output.
- Serious refactoring of many core parts of Redis.

Migrating from 3.2 to 4.0

- The Redis Cluster bus protocol of 4.0 is no longer compatible with
  Redis 3.2.
- Redis Cluster CLUSTER NODES output is now slightly different.
- Writable slaves do not propagate writes to their sub-slaves, so
  writes to writable slaves remain just local.
- The RDB format changed. Redis 4.0 is still able to read 3.2 (and all
  the past versions) files, but not the other way around.
- Certain log formats and sentences are different in Redis 4.0.
- Certain INFO fields, especially related to replication, are now
  different.
- GEODIST, GEOPOS and GEOHASH return values changed for non existing
  keys
- The SLOWLOG command entires contain additional two fields: the
  client address and name. This is documented in the SLOWLOG command online
  documentation.