This message was posted on behalf of Intel Corporation
Converting 4K videos is very intensive CPU task, it is expected that the processor uses all the resources to complete this task. You will see a increase on the frequency while converting any videos. One more thing you can take into consideration is that this processor TJUCTION is 100°C, and as long as your system does not crash or freeze it should be fine. If you are concern about the status of your processor you can check the health status of your processor by running the Intel® Processor Diagnostic Tool which you can find here; Download Intel® Processor Diagnostic Tool. This tool will help to diagnostic the status of your processor.
In regard to your thermal inquiry, I recommend getting in contact with Acer since they were the ones to decide what type of cooling solution include to their products and by this I mean they should have the right answers on what to expect on this specific scenario.