Is it possible to upgrade my license from standard to pro or do I have to buy a new license which will cost $4K?

Every IP supported on Cyclone V is available in Quartus Prime Standard. If you want to use OpenCL, you do NOT need an additional license for that starting from v17.0 and Quartus Prime Standard supports OpenCL just fine. If you want to use older versions, then you need an additional license. You will also need to get an OpenCL BSP from your board manufacturer or create one yourself; the latter is not provided by Intel except for their own reference boards. If you mention what board and what version of Quartus you are using, it would be easier to tell what you will need.