NOTE: Even though Derek Law 's link doesn't list/include 10.4.1, I just install the
Microsoft_SQL_Server_2012_SP3_Native_Client_64bit_149470 version of the client on my test 10.4.1 machine, and I was able to connect to what I believe is a SQL 2008 (SP4) SDE database....other than drawing something quickly, I did not test any further.
...this is critical for me before I upgrade too.