Inplace upgrade from SQL Server 2008 Standard to SQL Server 2014 evaluation
You cannot upgrade from SQL Server 2008 SP4 to SQL Server 2014 evaluation edition. Its not supported and wont allow you please see Supported Version and edition upgarde

You should not worry much about upgrading from 2008 Sp4 to 2014. The upgrade would not be a much of problem but how would application behave is something on which you should focus. You can download SQL Server 2014 evaluation edition , which is equivalent to enterprise edition for 180 days. Restore database from 2008 to 2014 and start testing. Don't use any enterprise features. When you are satisfied with application performance do an inplace upgrade to SQL Server 2014 standard

Before doing in place upgrade make sure you run upgrade advisor and mitigate all issues(if any) pointed by UG advisor.