I need a 4.5 inch arm

I'm afraid that we're not able to supply a version of the 39-187 in that size. There are two possible solutions you can try.

The first is to adjust the position of the mounting bracket on the sash to make up for the difference. I've done that myself and found that it works most of the time. There are some windows where this isn't possible, though.

The other option is to swap the link arms. You would drill the arms out on both the new and old operator, then install the old arm on the new operator using a 62-014 rivet. If you're handy then this is a relatively painless and guaranteed solution.

In most cases, you should be able to install the rivet with a hammer.