At Tyrolean, the DH8-300 and -400 are operated as a common fleet and on a common rating. As mentioned before by other posters, the simulator prof checks alternate between both subtypes, and you are required to fly a defined number of monthly legs on each variant to remain current.
When joining the fleet, you start on one subtype only and get the differences training approximately half a year later. This of course results in several crew members being rated only on one type, causing challenges for scheduling. Would be some surprise to find a plane for the return leg from a nightstop you are not rated to fly... But scheduling does a very good job on this; I have never heard of this happening.
The procedures are streamlined nearly completely, but there are still several things that one needs to remind himself of when switching subtypes. The maximum power setting on the -400 is 100% torque for 5 minutes, while the -300 has 105% for 10 minutes for example and starting the engines of the -300 the way it is done on the -400 would likely mean an instant hot start. There are some other items in that category, but you get an idea.
Pontiusīs copi is absolutely right with his comments. The common type rating has led to some artificial "dumbing down" of the -400; there are heaps of things the computer could easily do to take workload from you that were not found desirable towards that end.
In the end of the day, operation of the -300 and -400 on the same rating has its challenges, but it has proven feasible.
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