I have just completed an upgrade from the 4500 to the 5400 and am pleased with the results. The enhanced resolution is quite nice, the interface is far more modern and easy to use and you get some nice enhanced functionality.

In addition to what Rob mentioned you will also likely need to swap-out your Crossbow magnometer with an AHRS (since they are not built into the 5400). I upgraded to the VectorNav AHRS which is the exact same form factor as the Crossbow and uses the same connector and cable (so very simple upgrade).

You may also need to either install an inline resister in both of the pressure sensors (oil and fuel) or do what I did and replace the VDO sensors with Kavlico sensors that are a bit more accurate and reliable. However, this requires additional work because you need to run ground and power wire from the 5400 to the sensors (along with the existing sensor wires out of the 5400).

The biggest hassle was installing the new backup battery (unless you decide to run them directly off the main battery). The back-up battery is reasonable size and requires an additional 5amp CB to be added to the panel (or you could use an inline fuse). I managed to fit it just behind the 5400 MFD.

One final consideration is that if you have the AF-Pilot Autopilot and you want to see and hear it's status along with being able to engage and disengage it from the EFIS, then in addition to the ARINC 429 connection you also need to install the wires (RX and TX) for the RS-232 connection (and configure it on the associated 5400 PFD serial connection). I did not have the rs232 connection on my 4500 (and I know a couple of other folks with 4500s that don't have the rs232 connection) but since the 5400 has an autopilot status indicator in the middle-top of the display it makes sense to include this connection.