well i don't know the layout of your residence - but you need to monitor every first floor opening (doors and windows) AND any opening on any other floor accessible without the use of a ladder longer then 8 feet or so. I doubt 4 contacts will do that in any residence (i could be wrong - it depends on your situation) - i think that's where the 'add-ons' comes in. I would go with ADT - they usually have the top of the line hardware and their monitoring is top-notch. I had several estimates when getting my system installed - no one offered the hardware free - if they do offer it free - it probably comes with a clause that you have to go with their monitoring for so many years and it will probably be more expensive in the long run considering all of the discounts you're getting
Should have contacted the police and/or Fire Dept when you did it. To be honest you car is likely to be damaged far quicker then the hydrant. Also depends how you back up. Me I let the car do the work when I back up. If you lay on the gas and zoom back then yeah you may be more likely to do damage then if you go about 5 mph tops.