Can i put leak stop in the resevoir of the radiator

NO-NO-NO-NO NO

If you have to use stop leak drain out some coolant and remove the upper rad hose and poor it down there. IF you put it in the reservoir it will sit there and stop the return. You will think it is full and your system will be empty.

If you have 4.0 you may have a head gasket leaking. If it is running ok and you are loosing coolant contact me back and I will tell you a good fix that works

Ok I will give it a try thanks.

If you're going to use any stop leak put it directly into the radiator. I would suggest that you only use the powder type though, Bars leak will clog up the heater core. The powder type does work pretty well as long as the leak isn't huge. This is not a permanent solution to any coolant leak. A proper repair should be done as soon as possible. I hope this helps you.

You can, but it is better to put it straight in.
Typically you put it in when there is a leak, so the water level is low. If your water isn't low, there is no need to put it in.
If you are doing this as a preventative measure, it isn't a good idea - stop-leak is an emergency measure to get you home, not a permanent repair. If you have a problem with your radiator or hoses - get it serviced, it's a lot cheaper than a cooked engine.

Sure you can, that is the correct place to put it. But where do you have a leak?
I once had a leak in the heater core of a Ford and you won't believe how I repaired it. My father-in-law said to pour a small can of black pepper into the radiator and it would stop the leak in the heater core and it did. I never had to replace the heater core in that car.