The main difference between Bissell 17N4, 17N49 and 17N4P carpet cleaners are the Nozzle and the tools they include.

1, Included Tools
The 17N4 comes with a 3-inch Tough Stain Tool for cleaning stairs, furniture and other hard to reach surfaces, and a 4-inch Deep Reach Pet Tool for tough to clean carpet stains
The 17N4P comes with a 6-inch Stair Tool for cleaning stairs, larger furniture areas and other hard to reach surfaces, and a Stain Trapper Tool for removing extra-gross, extra-stinky messes.
The 17N49 comes with a 3 in 1 Stair Tool for removing dirt and stains from the outer curves, corners and treads of carpeted staircases, and the Stain Trapper Tool

2, Nozzle
The 17N4 has a standard nozzle.
The 17N4P and 17N49 feature a Durastrength Pet Nozzle – reinforced front nozzle to hep prevent cracks, breaks, stop pet hair and debris from getting clogged in the nozzle.

The other difference are the color and included trial size formula.
The 17N4 comes with 8 oz trial size formula while the 17N4P and 17N49 come with 24 oz cleaning solution

If you know any other differences between these 3 carpet cleaners, please share below.