[kutter]

not specifically for omniplan, but I dont know how to setup a task.

Lets say I have a task that takes me 2 hours to complete, and I can do it anytime in the next 4 days. How do I represent that in ominplan? Right now, I choose 2 hours for effort or duration, but the start and end dates are wrong, or I choose start and end dates and the effort is 4 days.

[Lizard]

Set up the task's duration as 2 hours, and add an end constraint* for the deadline. The start and end dates are wrong. Why are they wrong? If they're wrong because you're not working on this project for the next couple days, then you should tweak either your work schedule or the project schedule in the calendar view.
If they're wrong because you're working on other tasks for this project first, then you should add those tasks to the project and level, so that OmniPlan can calculate when you actually will get to this task.
You can also use 'reschedule' as the project progresses, to bring incomplete tasks up from the past automatically.

*Often, adding an end constraint is a sign that you've actually left another task or milestone out of your project. Why does this task need to be done in the next four days? If it's so that you can get on to the next phase of the project, a finish->start dependency might be more appropriate. An externally imposed deadline maybe should be a milestone.