Why a 100 Mbps plan isn't 100 MB/s
Internet speeds are advertised in megabits per second (Mbps), but file sizes are measured in megabytes (MB). There are 8 bits in a byte, so you divide the speed by 8 to get the real-world download rate. A 100 Mbps connection tops out at roughly 12.5 MB/s, so a 4 GB file takes around five and a half minutes at full speed.
Real-world speeds are lower
These figures assume you get the full advertised speed with no overhead. In practice, Wi-Fi, network congestion, the server you're downloading from, and protocol overhead all shave off a chunk. Treat the result as a best-case floor for how long a transfer will take.
Curious how much data those downloads add up to each month? Try the data usage estimator, or see how much internet speed you actually need.