The download speed won't be limited by your local LAN but will depend on the speed of your internet connection. Networking is an end-to-end thing, and like a chain is only as strong as its weakest link your overall connection will only go as fast as it's slowest element
what you need to do then is to identify the elements that make up that connection and see which of them is slowing you down. For example, if you simply copy a file off the webserver machine rather than download it, do you get the same throughput? What happens if you copy that file off a different server? And what happens if you use a different PC to copy/download the file from the webserver? By doing tests like these you shopuld be able to identify where the bottleneck lies. Once you know where it is, then the fun begins!
there are a couple of things which could cause this.
Your 100 Mbps = 100 Mbit/sec.
Your Windows says 3.8 MByte/Sec
100Mbit = 12,5 MB/sec.
your tcp/ip uses some ip-windows to send data from one side to another... which results in having 75% usage of your 100Mbit.
Resulting in 9,3MB.
As your computer builds from 0MB/s to 4MB/s.. it also takes a bit time.. so all by all your 3.8MB/s isn't that bad it all. Maybe you also have some other data from your computers/servers which results in more utilization.