HP MFPs do not co-exist very well with other MFPs. This is because the scanning software tends to be shared across the different printers and can cause issues when one tries to update a shared resource. It is always advised that you install and use only one MFP on a computer at a time to avoid these little conflicts.
Start by wiping up all your MFP software on your computer. When you are done rebooting then run the installer for your LJ400 again. Wait for the install wizard to tell you its OK to plug in the printer if you intend to use it over USB. Otherwise be sure to select the network installation option so you get the correct install options.