Cardinality Estimator Question after 2014 SP1 + CU4 Upgrade
Based on my experience with such issue and as mentioned in This Blogs.msdn article 


  You need to apply SP1 but you must also enable trace flag 4199 in order to activate the fix.SQL Server 2014 Service Pack 1 made various fixes on new Cardinality Estimator (new CE).  The release notes also documents the fixes.


So I suggest to suppress regression you enable it.

Trace flag 9481 solved your problem because if forced SQL Server to use old optimizer. So basically you are using SQL Server 2014 but not new CE which comes with it.

Trace flag 4199 makes sure the optimizer is using all changes/fixes made to SQL Server optimizer since SQL Server 2005.

From this support article


  Trace flag 4199 was used to collect hotfixes that were intended to become on-by-default in a future release, whereas other trace flags were used for situations in which a fix was not intended to become on-by-default in current form. Starting with SQL Server 2016 RTM, the database COMPATIBILITY_LEVEL setting will be used enable trace flag 4199-related hotfixes on-by-default. This article describes the mechanics and policy of how plan-affecting hotfixes will be delivered for SQL Server 2016 and later versions.


There are three ways to do it depending on need.


By enabling the trace flag in a batch (by using DBCC TRACEON command) right before the target query, and then disabling the trace flag (by using DBCC TRACEOFF command) right after the query.
Starting with Microsoft SQL Server 2005 Service Pack 2 (SP2) and Microsoft SQL Server 2008, the query-level option "QUERYTRACEON" is available. This option lets you to enable a plan-affecting trace flag only during single-query compilation.
You enable it in starup parameters.


The first and second point will only make sure query/batch runs with 4199 trace flag while third point makes sure any query which runs sees this trace flag enabled.

NOTE: Enabling trace flag requires sysadmin permission so if you ask your developer to use it in query for instance he might not be able to assuming developers have limited access.

I would suggest you to use querytraceon or dbcc traceon to see if queries are performing up to mark.