Help partitioning "free space" with utility disk
First I have to explain the gpt result:

The partition with index 1 is your EFI partition. The partitions with index 2 and 3 are your main OS X volume named Macintosh HD (also visible in the second screenshot) and your Recovery HD. The partition with the index 4 is your NTFS volume WinB (also visible in the second screenshot).

Trying to add a new partition (by using the +-button) or to resize Macintosh HD (by using the handle) with Disk Utility will downsize partition 2 and move partition 3 to the new end of partition 2.

Since adding a new partition with the +-button doesn't work properly for you, you have two options:


use the handle and resize Macintosh HD. This should create unallocated free space
or use the +-button and create a new volume. Apparently it will be formatted as a HFS+ Journaled volume.


Then boot to Internet Recovery Mode and open Terminal. 
Enter diskutil list and gpt -r show /dev/disk0.  
Unmount your main disk with diskutil umountDisk /dev/disk0.
Remove the new HFS+ volume with gpt and its index number found above:
gpt remove -i 4 disk0
Enter exit and quit Terminal
Reboot



This might break Boot Camp as stated in Disk Utility and is not recommended if Windows boots in MBR-mode. So backup your Mac (including the Bootcamp partition) first.



I tested this with a spare MacBook Air (Mid 2013 with a 250 GB SSD) and a vanilla Yosemite after installing Windows 8.1 R1. Regardless what I did, I had no problem to boot from either Windows 8.1 or OS X. Windows 8.1 seems to be booted in EFI-mode though.

Since you have an older Mac, Windows will probably be booted in hybrid MBR-mode. Adding a fourth partition (at position 3) will break bootability of the last one because only three partitions (and the EFI partition) are allowed.