Raid spare or Raid Slice

What is the best option ? Slide or Spare ?

Depends on what you want.

A spare disk is unused until another disk in the array fails, a which point the system immediately starts rebuilding the data on the spare disk (that's what it's there for).
A slice is part of the array - it's capacity is added to the array.

So do you want capacity or redundancy?

I presume that your 4TB is made up of four x 1TB drives.

In that case you can't use just RAID 1 or RAID 0

RAID 1 would mirror all the drives - give you four copies of the same 1TB of data
RAID 0 would stripe all data across all drives, giving your 4TB of space, but no redundancy (lose one disk and you lose everything).

What you want is RAID 1+0 (sometimes called RAID 10). For this you first create two mirror sets (so drives 0 and 1 become one mirror set, drives 2 and 3 create another), you then create a stripe array using both the mirrored volumes as the components of the stripe.

This will give you the combination of 2TB of space on mirrored drives. There are no spare drives, though, so a failed drive will need to be replaced manually.

PS : I suppose I'll just have to type RAID 1+0 in the field "RAID Set name".

You can type whatever you want. It's just a label.

why the application RAID utility is unavailable in my iMac ? When I click on the icon, it says "RAID utility is not suppported on this computer".

RAID Utility is for controlling the hardware RAID card in an XServe (or MacPro). Your machine does not have a RAID card, therefore it is not needed.

Everything described here is software-RAID and can be done via Disk Utility (although I'm not sure how you're getting 4 drives in an iMac... 🙂