I don't think a Mac Pro can do this job better than a NAS. The speed of NAS cannot higher than your network speed. And most likely it will max at 125MB/s (1000Gb/s). In real world, usually max at around 110MB/s. Most modern HDD can easily achieve this speed limit, and you don't need a very fast CPU to deliver data at this speed.
Of course, a very fast NAS with 4 HDD slot + has RAID function may cost you lots of money. Use your Mac Pro as NAS is not a bad idea, because it almost guarantee that you can achieve the max NAS speed. However, it will draw much more power than a normal NAS, so you have to pay more for electricity, and may be end up no difference than buy a fast, new, quiet NAS.