This will probably happen whenever you upload anything from any machine in your home - downloads require upload bandwidth to acknowledge the downloaded data, if the upload bandwidth is limited the acknowledgements can't be sent in time and the download will become slow. Most ISPs only offer enough upload bandwidth to cover these acknowledgements, so any uploading kills your download.
You might want to investigate QOS (quality of service) options on your router to see if you can limit the PS4's bandwidth a little.
Blargh! QOS has limited access, and with the regular admin account, it doesn't show. Oh well.
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