In theory, your transfer speeds should be 4x as fast as a single similiar drive, but in practice you probably see something like 3-3.5x the speed of a single drive (maybe less if you are limited by your controller). Your access times will remain identical to that of a single similiar drive. Aside from speed, there are 2 other major considerations putting those 4 drives in a RAID 0, reliability and partitioning.
A 4 drive RAID0 array will be 4x as likely to fail as a single similiar drive. If any one of those drives dies, your entire RAID0 partition will be lost (there are ways to recover the data but it's not pretty and they require that the "dead" drive be at least mostly readable). You said you're coming into posession of those 4 drives, are they used? This is a major consideration since the array is only as reliable as the least reliable drive int he set.
If you put 4 750s in a RAID 0 you'll end up with a 3TB array. You will either need to partition this volume into 2 seperate partitions of no more than 2TB each or use GPT partition if you are running windows. GPT partitions are only bootable if you have a UEFI BIOS.
If I were you I would take 2 drives and make a RAID 0 array out of 2 of the disks, and partition into (2) 750GB partitions. Take the other 2 disk and create a 750GB RAID 1 array. Install your OS on the first RAID0 750GB partition and setup a weekly job to image the boot partition to your RAID 1 array. This will give you 750GB of fast storage that is also backed up in the event of a failure as well as 750GB of extra storage that would not be backed up.
Additionally you could create a RAID10 array which will give you same the same performance and space as a 2 disk RAID0 but will protect against a drive failure. I didn't recommend this solution though because it is not a true backup as you cannot "undo" something at the software level such as deleting files, a virus etc.