partitioning - sd4 is nested! for example sda5 is a child of sda4! Ubuntu does not allow me to create any other partition!

Below pictures (captured form gparted) show my problems!

sda1 -> Windows system reserved (created automatically by windows os) -> PRIMARY
sda2 -> Win 8.1 -> PRIMARY
sda3 -> Ubuntu -> PRIMARY
sda5 -> swap as a logical partition, marked as 'Beginning'
sda6 -> home partition as a logical partition, marked as 'end'

Now, there is a little free space that I want to install a new linux distro on it, but When I try to create a new extended-logical partition I get this error: Cannot create a new partition. There are already four primary partitions.

Why ubuntu created sda4 while I had choosed 'logical' for both swap & home partitions.

How can I get rid of sda4 and make a new partition?

• More details are here:

sudo parted -l

Model: ATA ST3500412AS (scsi)
Disk /dev/sda: 500GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number  Start   End     Size    Type      File system     Flags

 1      1049kB  368MB   367MB   primary   ntfs            boot
 2      368MB   70.6GB  70.2GB  primary   ntfs
 3      70.6GB  101GB   30.7GB  primary   ext4
 4      101GB   463GB   361GB   extended
 5      101GB   104GB   3071MB  logical   linux-swap(v1)
 6      104GB   463GB   358GB   logical   ext4

sudo fdisk -l

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xd42cd17a

Device    Boot    Start       End         Blocks    Id    System
/dev/sda1   *        2048      718847      358400    7    HPFS/NTFS/exFAT
/dev/sda2          718848   137912319    68596736    7    HPFS/NTFS/exFAT

/dev/sda3       137912320   197912575    30000128   83    Linux
/dev/sda4       197914622   903913471   352999425    5    Extended
/dev/sda5       197914624   203913215     2999296   82    Linux swap / Solaris
/dev/sda6       203915264   903913471   349999104   83    Linux

You can create only 4 primary partitions(including extended partition) in a disk containing MBR partition table.You can create any number of logical partitions inside extended partition.

But in GPT,you can create upto 128 partitions.There was no primary or logical partitions in this type of GPT disk.

From your question:

When I try to create a new extended-logical partition I get this error: Cannot create a new partition. There are already four primary partitions.

It was false,you try to create a new partition on the second unallocated space which was outside the extended partition.You had already four primary partitions,so that the error appears.See the below screenshot,

But you can be able to create a new logical partition on the first unallocated space which was inside the extended partition.

Why ubuntu created sda4 while I had choosed 'logical' for both swap & home partitions.

Logical partitions are always comes inside the extended partition.You choose swap and home partitions as logical,so it comes under extended partition /dev/sda4.

How can I get rid of sda4 and make a new partition?

You can't be able to delete sda4 because your home and swap partitions are inside on that sda4 extended partition.