SQL Server CUs vs SPs + CUs?
Should I update it to the latest SP + CU (SP3 CU4)?


When a new SQL Server service pack is released, Microsoft provides only 12 months of full support for the previous service pack.

Support is limited for older SPs:


  For customers on supported products with service pack versions that
  have left full support, Microsoft offers commercially reasonable
  support as follows:  Commercially reasonable support incidents will be
  provided through Microsoft Customer Service and Support and Microsoft
  managed support offerings (such as Premier Support). If the support
  incident requires escalation to development for further guidance,
  requires a nonsecurity update, or requires a security update,
  customers will be asked to upgrade to a fully supported service pack.


. . .


  Microsoft recommends that you apply the latest Cumulative Update (or
  one released in the last one year) for the corresponding release. The
  support team might require you to apply a specific Cumulative Update
  that addresses a specific problem when troubleshooting an issue.


https://support.microsoft.com/en-us/help/17138/service-pack-lifecycle-policy

So cross-referencing the release history here, yes you should apply SQL Server 2014 SP3 CU4.