Be quite familiar with the game, its mechanics and the mission layouts. Do not start after only one playthrough which is the most difficult in DMC4. The run difference between each characters are significant and are hard to be consistent at especially with Nero and Dante. The further you progress as you keep playing DMC4, you'll find yourself get in a very long ride.
The excessive resets can be one big issue for most players Especially for early missions, in which can be difficult to go through with no damage requirement pressure after getting hit from Mission 1 and 2 bosses. Therefore, take your time and refer to different perspectives over what to do and whatnot. Do not focus too on one thing which can burn you out very fast. Make your own theory-crafting strategies and get familiar with the Terminologies and safe methods that are used during the runs.
In the following sections, you will find important concepts and information that you may find useful for starting out with DMC4 Speedrunning.
Refer to DMC4 main page for general tricks being used along with tutorials here: