I have 3 OSes and and partitions are as under

1. I wish to utilize unallocated space for fourth OS. can I

   Can I merge unallocated space with extended partition so I can creatye more logical partitions

My machine is 64-bit , Intel i3
one os is ubuntu 12.04 64-bit
other is ubuntu 11.10 32-bit
and one windows
FDISK - l yields as below

Disk /dev/sda: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders, total 1953525168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0xe7f191d1

Device Boot Start End Blocks Id System
/dev/sda1 * 2048 206847 102400 7 HPFS/NTFS/exFAT
/dev/sda2 206848 204799999 102296576 7 HPFS/NTFS/exFAT
/dev/sda3 204802046 415737855 105467905 5 Extended
Partition 3 does not start on physical sector boundary.
/dev/sda4 415739904 1323034623 453647360 83 Linux
/dev/sda5 204802048 302456831 48827392 83 Linux
/dev/sda6 302458880 318081023 7811072 82 Linux swap / Solaris
/dev/sda7 318083072 415737855 48827392 83 Linux

If you wish to delete /dev/sda4 and enlarge the extended, yes you can. Presuming you don't want whatever is in sda4
Best done from a gparted liveCD IMHO.

You dont have any free partition in your hard disk. Dont delete /dev/sda4 which will lead to system breakdown of your Ubuntu. You can check the extended partition /dev/sda3 inside every OS. Might be it is used some where else as a mount point. if not you can use that .

Your listing does not reveal which partitions are free. Post something like content of /etc/fstab for all your distros (Unfortunately I don't know what is the equivalent for win). All your partitions are listed as if prepared for win/linux, although some of these may actually not be used.

In most cases you can merge consecutive partitions into a single larger one. If they are not located next to each other, that may be tricky (you have to move/resize the file system).

In long run I recommend you to look at lvm, it adds a lot more flexibility in therms of multiple partitions/harddisks, and gpt partitions (you don't have the 4 primary partitions only limit).

If I understand correctly:

You are using three Linux partitions: sda4, sda5 and sda7 for two Linux installs.

You have unallocated space after sda4, which can't be used as things currently stand.

While sda4 is not mounted, you should be able to move sda4 to the physical end of the drive so the unallocated space is before it instead of after it.

Then, while sda5 and sda7 are not mounted and the swap space in sda6 is not active, you should be able to expand sda3 to include the contiguous unallocated space.