Just upgraded to the MiFi 4510 and my download speeds at best are the same as my 3G modem and at worst 1/2 the download speed. I have a 4 bar 4G signal and I am square in the middle of 4G coverage. Download speeds are 800kbps to 1.5MBps
Are you sure you are connected to a 4G signal where you are having this problem?
The easiest way to tell is to look at the LED light on the side of the MiFi. Green = 4G and Violet = 3G. All 4G areas are also in 3G areas so your device might not be picking the correct one. I average above 5Gb for download speeds when I am connected to 4G.
In case you are not already doing so, try connecting to www.speedtest.net the next time you notice this problem. Run the basic test a few times and record your numbers. That should give you a fairly standard way of gauging your connection speeds.
we had the exact same problem and this resolved the speed issue.
Give it a try as you can always set it back.
Force the MIFI to only speak Wireless G and not use N. when doing this we went from 1MB up to 10MB on speedtest results.