Four-bolt toilets aren't used anymore, so if you have one that isn't working, you'll have to replace it with a two-bolt model. You shouldn't have any difficulty, because the same two bolts connect the toilet to the flange on both types of toilets. The two extra bolts in the older models are usually screwed to the floor. After you remove the old toilet, make sure the flange is at the correct distance from the walls to accommodate the new toilet, and that it is corrosion-free and at the appropriate height. You should also check the condition of the subfloor.
Turn off the water supply and flush the toilet. Hold the handle down to empty the tank, then unscrew the water supply hose with a pair of slip-lock pliers. If the toilet has a metal supply tube, take the opportunity to unscrew it from the supply valve so you can replace it with a braided one.
Unscrew and remove the nuts from all the bolts with a wrench. Lift the toilet straight up off the floor until it clears the bolts, then set it aside on a plastic sheet. Sponge out the water or get someone to help you carry it outside where you can turn it over to dump out the water.
Clean the wax off the toilet flange with a putty knife and collect it in newspaper so you can throw it away. Stuff some newspaper in the waste opening to block sewer gases and leave it there until you are ready to install the new toilet.
Unscrew the bolts that aren't attached to the flange from the floor with slip-lock pliers. Unhook the two other bolts from the flange and throw all the old bolts away.
Measure the distance of the center of the flange from the wall to make sure there is room for your new toilet. The standard distance is 12 inches, but some models may require more or less clearance. Check the specifications for your model.
Hook 2 new toilet bolts into the flange slots and center them so they are lined up across from each other in the flange. Remove the newspaper from the flange opening and place a wax ring, tapered side down, in the the flange opening.
Lift the new toilet into position and over the flange and line it up so the bolts fit through the holes in the bottom. Lower it onto the wax ring and push down on the bowl to compress the wax. Drop washers onto the bolts, then screw on the nuts and tighten them with a wrench. Tighten each one a little at a time rather than fully tightening one and then the other. This will keep the bowl balanced and prevent breakage.
Screw a new braided water supply hose to the water valve and tighten it with a wrench, then screw the other end to the tank inlet and hand tighten it. Turn on the water, and as the tank fills, check for leaks from the tank. Flush the toilet after the tank fills and check for leaks under the bowl.