Sounds Like you internet connection is unstable. Internet providers often Throttle their connections too especialy 4G providers. Some packets have failed to download or got corupted during an interuption in your connection. If their is a large load on the ISP’s network they will have a max transfer rate set. Your downloads will peek at the the max rate then drop back to nearly nothing. This constant throtling of your connection is fine if your on youtube but if your downloading 3.3 gigs its going to mess things up. Try to use a fast wired connection when downloading. I often have to move my intire PC a few hours down the road to a friends house, just to install stuff like UE4