I love Reba, but I also agree it probably wouldn't have made it on one of the big four.
The problem was Reba was very inconsistant the first year. You can see the producers weren't sure Reba could pull off comedy, so they put Park Overall, as Reba's friend as the comedy.
But Reba quickly proved she can do comedy and do it well. Then when the restructured Barbara Jean from being "BJ" (making jokes about that name, note after a few episodes they droped BJ for Barbara Jean) and toned Barbara Jean down from a psudeo holier than thou Christian, to wanting to be Reba's buddy.
When they put Melissa Peterman and Reba together, the chemistry was outstanding.
And that didn't really happen till the end of the first year. Those two together played off of each other perfectly. You'd have to go back to "Kate and Allie" or "Laverne and Shirley" to find two actress playing off each other comedically as well as Melissa and Reba.
And that chemistry between the two made Reba good. Without Melissa, the role of Barbara Jean wouldn't have been what it was and the WB gave Reba enough time for Peterman to show what she could do.