Dave Filoni is working on the next show, so Rebels is probably not going to last more than 3 seasons (or maybe a few more set between ANH and TESB ?). With Rogue One coming the ANH “direct prequel” era has enough material, could be fun to explore other eras, such as after ROTJ.
This is almost certainly not the last season. I agree with Tobar, five sounds about right (and would get them right up to ANH if they’re doing one season = one year).
And Dave Filoni working on a new show is just speculation at this point, though it wouldn’t be surprising if true. Either way I’m hoping the next show is set immediately post-ROTJ.