Are you asking to create a variable that only carries 3 or 4 digits of precision? You can't choose to set an arbitrary precision, unless you use a tool like my HPF, but that would take far more effort on your part to learn than it would help you, and it has limits.
At best, you can use single precision, giving you roughly 8 digits. And Cleve Moler has posted some tools for shorter prescision, but those tools will also be limited in which functions can use them.
Or, are you asking how to plot a function over a domain that varies over 3 or 4 orders of magnitude for a plot?
That is far easier, since you can use tools like logspace, or semilogx, etc. And of course, you can always work using logs.