It seems like that the 4.6.* installer auto-uninstalls any 4.5.* versions on MacOS, I can replicate this.
A workaround:
Reinstall 4.5.7. Then go into /Applications/Dynare and move the folder 4.5.7 to some other location. Then install 4.6.4 and move 4.5.7 back to /Applications/Dynare.
