As far as I know, there are some differences between Bissell 36Z9 and 17N4:
The first one is the Cleanshot feature, the Bissell 36Z9 equip with the Cleanshot trigger for tough stain cleaning while the 17N4 model does not have this function.
Bissell 36Z9 Cleanshot:

The second one is the tool they come with, 36Z9 come with a Stain Trapper tool while the 17N4 include a Deep Reach Pet tool.

Stain Trapper tool:

Deep Reach Pet tool:

The third one is edge brush. If you look closer, you can find that the 36Z9 has wider edge brush.

36Z9 edge brush:

17N4 edge brush:

The fourth one is the nozzle, I read somewhere that the 36Z9 has a metal reinforced front nozzle.
36Z9 nozzle:

17N4 nozzle:

Therefore 36Z9 is more expensive than the 17N4.

	Bissell DeepClean Premier Pet Carpet Cleaner 17N4	BISSELL DeepClean Deluxe Pet Carpet Cleaner 36Z9
Official Price (2014-04-18):	$229.99	$299.99 36Z9 is $70 more expensive than the 17N4 on bissell.com.
Cleanshot	No	Yes 36z9 feature a Cleanshot Trigger that sprays the formula directly at tough stains for spot stain removal.
Color	Silver B123 with Grasshopper accents	Silver B123 with Passion Purple accents
Tools & Attachments	Deep Reach Pet Tool, Tough Stain Tool	Stain Trapper Tool, Tough Stain Tool
Cord Length:	22 feet	22 feet
Tank Capacity:	1.25 gallons	1.25 gallons
Brush System:	12 Row Dual Dirtlifter® PowerBrush	12 Row Dual Dirtlifter® PowerBrush
Surface Cleaning	Carpet, Upholstery, Stairs, Low Pile Carpet	Carpet, Upholstery, Stairs, Low Pile Carpet
Cleaning Path Width:	12"	12"
Formula Included:	Trial-size 2X Pet Stain and Odor Remover Formula (8 oz)	Trial-size 2X Pet Stain and Odor Remover Formula (8 oz)
Weight	24.4 lbs	24.8 lbs
Warranty	3 year limited	3 year limited
User Guide	17N4 Manual	36Z9 Manual
Official Link	http://www.bissell.com/deepclean-premier-pet/	http://www.bissell.com/deepclean-deluxe-pet/

If you know other difference between these 2 Bissell carpet cleaners, please share, thank you.