i have a sondors bike that comes with a 4.0 fat tire. i am tring to find a street tire for my fat boy i only found one and its 3.5 not 4.0 STREET TIRE
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.
What matters is the width of the rim. It's unlikely that you rim is 4" wide, so you will be fine with a 3.5" tyre.
If you search for "narrow tyre" or "tyre width" here you will see quite a few questions about this, and the very general rule is that the rim should be narrower than the tyre. But I think yours is the first fat-bike question.