I got them off and replaced.

two of the four bolts I was able to work out by using an impact and going loosen and tighten over and over again with generous use of Liquid Wrench.

One I had to burn the rubber out and make an effort to open the metal tube with a chisel. That one took some pounding with a two pound "gentle persuader". Again in and out until it loosened enough to force out with a punch.

The final one was the most difficult. The hole in the frame allowing access to the bolt head did not leave much room. The exhaust is just too much in the way. This one would have been easier with the right combination of extensions and sockets. I was able to work it in and out with the hammer and a pry bar until it finally came free enough for the punch to work.

Installation was simple and straightforward the new parts were in and on in less than 15 minutes total.

New bolts are almost a requirement after 18 years use here in the northeast. The winters and road salt really do a number on the bolt heads and the nuts.