While you can control how Project does the calculation when you add or
remove resources or edit the assignments of resources that are already
assigned, there is no way to manipulate duration and work completely
independently. Project's "Prime Directive" is W=D*U, Work equals Duration
multiplied by Assignment Units, and it is absolutely impossible ever to have
any tasks where this identity is violated (that applies to tasks out in the
real world as well as to tasks in the project plan, for that matter, it's
something fundamental to the physics of work).
Here's what you can do. First of all you have to decide what you mean when
you say the task is a 4 hour task. Does that mean that it would take ONE
person devoting their full attention to it 4 hours to complete the work or
do you mean that your TWO people working together will take 4 hours? If ONE
person will take 4 hours, will adding the SECOND person get it done in 2
hours or will it still take 4 hours (a meeting, for example, doesn't get
shortened when we add another person to attend)?
Next, decide what you mean by "needs to be done over a three days period."
Is it something where the work must be spread out in drips and dabs over 3
days (an example might be an automated test of some sort that runs 3 days
and needs a technician to spend a few minutes every hour taking a reading),
or is it something that will take 4 hours to do once it starts but you have
3 day "window" over which you could get it done? If it's the former, and for
simplicity looking at just one resource, 3 days duration is 24 working time
hours, so 4 hours work distributed over 24 hours of duration means the
resource is assigned at 4/24 or about 16% assignment units. Over the course
of 3 days, out of every hour he's at work he puts in an average of 10
minutes work on that particular task. If it's the latter, I wouldn't
distribute it at all. Make the task 4 hours duration, assign the resource
to it 100%, meaning he's execpted to work full time on it once it starts,
and set a deadline of 3 days after it starts to show its required finish.
That way you'll "git 'er done" as soon as possible and your schedule will
have a 3 day cushion to help absorb the uncertainties and some of the
inevitable delays that creep in when you do the work.
