I think it depends on if the parlay rate is set or not.
For places like Intertops and William Hill, there is no set rate for parlays so you just multiply the odds together.
Using the decimal system, if you pick 4 games at 1.9 (which is -110), then you take 1.9^4 (1.9*1.9*1.9*1.9) which equals 13.0321.
So $10 X 13.0321 = $130.30 which is 12 to 1
This will be off a little bit of course because sometimes you get odds that are not 1.90. For example, this Sunday Night game is Green Bay -3 at 1.86 on William Hill.
This is my parley for sunday, see what you think
New Orleans +4 1.9
Pittsburgh -4 1.9
Buffalo +1 1.9
San Diego -4 1.9
All 1.9 pays $130.30 for $10 (as stated above)
You can see the coupon here.
BlackCoffee