It has been a while already, but the difference training from -300 to -400 consisted of a 4 days theoretical course followed by a few simulator sessions and IIRC 5 legs of supervision in my company.
However, the -300 has been phased out here in the meanwhile, so no more of these courses are done. One company that still operates both variants is Wideroe - You might want to give them a call. Or contact DHC at Toronto straight away; they might be of some assistance as well.
One word of warning: in my opinion, flying the -300 and the -400 under one type rating is not without difficulties. The aircraft may have superficially similar systems, but limitations, system operation and handling differ significantly in detail. Among others, power settings especially in the OEI case, engine startup procedures, flare height, descent planning and also runway performance are rather different. A little spreadsheet showing significant numbers and data of the -300 vs. the -400 is a valuable help and best kept close at hand at least during the initial stage of dual operation.
Last edited by Tu.114; 30th October 2013 at 15:39.