The Sims 4 48 Hour Free Trial

These might help:

Download is really slow despite of fast Internet connection 

This is usually the case if there's lots of people downloading at the same time. You can try pausing the download and resuming to see if it picks up in speed.

Download reverts to lower percentage

Try turning on Safe Downloading in your Origin Options (click on the spoiler below to view an exemplary screenshot).

Source: http://answers.ea.com/t5/The-Sims-4/FAQ-Serial-Codes-Game-Activation-and-Download-Issues/td-p/4228232