Yeah MC, don't stress! You will be doing your sim check on the 146. Nothing difficult, most likely a departure, some GH, and an ILS.
In the 'good old days' new starters used to do their initial on the 200/300. Much easier to fly (it all goes so much slower!), and then convert on to the 400 as and when.
New starters have been joining straight onto the 400 for almost a year now, and there don't seem to have been many problems with it.
Good Luck