Can I fit a 26x3.5 tire on a 26x4.0 rim?
A 26"x3.5 tire should work on any narrow 4" fat bike rim.  That is: 60mm-70mm wide rims.  It might work on an 80mm rim depending on the actual width of the tire. But will definitely not work on a 5" fat bike rim which is 100mm wide.

Surly does not recommend their Black Floyd (listed at 26"x3.8" but actually narrower) road tire for rims wider than 65mm, for comparison.

I tried to find the actual specs for a the rims on a Sondors Fat eBike, but it looks like there are a couple versions.  One comes with 59mm rims, which should work for a smaller tire.  However, the other version (with the 4.9" tires) might have a rim that is too wide for those tires. 

The Apache Fatslick http://veetireco.com/listings/fat-tire-apache-fatty-slick/ is the only option I know of a "road" tire for a 5" rim.