I have a Quad Core and 4 tasks are run every time... but only one is showed on screen... so I would like to have just one task run at a time.

And leave the other 3 cores unused?

Can be done through your preferences.
1. Web based preferences, go to the web site of the project you run, your account, computing preferences, edit preferences.
-> for BOINC 5 and earlier, set amount of CPUs through On multiprocessors, use at most x processors. Set this to 1.
-> for BOINC 6 and above, set amount of CPUs through On multiprocessors, use at most x% of the processors. Set this to 25%.

Save changes to the web site.
Open BOINC Manager->Advanced view->Projects tab->Select the project you just edited your preferences on->click Update to download the new preferences for BOINC to use.

2. Local based preferences (only available in GUI form in BOINC 5.10 and above).
BOINC Manager->Advanced view->Advanced->Preferences.
-> for BOINC 5 and earlier, set amount of CPUs through On multiprocessors, use at most x processors. Set this to 1.
-> for BOINC 6 and above, set amount of CPUs through On multiprocessors, use at most x% of the processors. Set this to 25%.

Click OK. Changes take immediately.

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What I want is to use 4 cores to run same task. So it will go faster.

BOINC can only crunch one task per CPU. It can't do parallel processing.
There are other ways to speed up work, though, but that depends on which project you crunch for. Some projects use so called optimized applications, where the application has been compiled to use the SSE/SSE2/SSE3/SSSE3/SSE4.1 code additions of the CPU. You can usually find threads to that effect in the Number Crunching forum of that project.

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OOOOOoohhhhh.... hope next version does it...

Well, If you have the option, you can choose.. I would prefer to have all cores crunch same task and see it on the screensaver, running so fast and clear.

For tasks that take 10 hours or less, I can't see the point of running 1 of them on 4 cores. [...] How is the end result different from running 1 on 4 cores?

Dag, do not forget our beloved memory pigs. Imagine over a gig of RAM per task on a quad core 32-bit Windows machine, and you'll get the point and an answer to your question.

Peter