[Galaxy Tab 4 Q&A, Help & Troubleshooting] Slocum0217:
I had exactly the same problem (black screen "Power Upload Reason:Power Key" and below that "H/W Revision : 0.3"), due exactly to the same reasons (following the advice

mke2fs -T ext4 /dev/block/mmcblk0p8

, in order to sort out "Unable to mount" ), with exactly the same device (SM-T230NU), and yesterday I could sort it out, with some luck. It's working perfectly now.

I'm not an expert at all

, not even close. I just tried to do some research and I got some luck. Given the status of your device, I'd say you do not have much to lose if you give a try to this.

What I did:

1. I stopped trying for a while, perhaps a couple of hours with the device power turned off ( I don't know whether this is important or not; it's just what I did)

2. After this while, I connected my tablet to my PC via USB (by the way, I have android SDK installed in my PC, in order to be able to use adb), tried to turn on the tablet in recovery mode (TWRP, the same you have), and surprisingly recovery mode appeared.

3. I made sure not to lose this connection: I didn't know if I would ever be able to get in recovery mode again.

4. I pulled out (with adb) recovery.log to PC. After many lines in that document, I found a section beginning with "Partition Logs:" which clearly describes which "mmcblk" to which partition. For instance, mmcblk0p9 belongs to recovery

[COLOR="Red"]/recovery | /dev/block/mmcblk0p9 |[/COLOR] Size: 16MB
Flags: IsPresent Can_Flash_Img
Primary_Block_Device: /dev/block/mmcblk0p9
Display_Name: Recovery
Storage_Name: recovery
Backup_Path: /recovery
Backup_Name: recovery
Backup_Display_Name: Recovery
Storage_Path: /recovery
Current_File_System: emmc
Fstab_File_System: emmc
Backup_Method: dd

With reference to 'cache' (mmcblk0p14) and 'data' (mmcblk0p16) partitions, I noticed that size, used space, free space available and backup size was zero for both of them:

[COLOR="Red"]/cache | /dev/block/mmcblk0p14 [/COLOR]| [COLOR="SeaGreen"][B][U]Size: 0MB Used: 0MB Free: 0MB Backup Size: 0MB[/U][/B][/COLOR]
Flags: Can_Be_Mounted Can_Be_Wiped Can_Be_Backed_Up Wipe_During_Factory_Reset Wipe_Available_in_GUI IsPresent
Primary_Block_Device: /dev/block/mmcblk0p14
Display_Name: Cache
Storage_Name: Cache
Backup_Path: /cache
Backup_Name: cache
Backup_Display_Name: Cache
Storage_Path: /cache
Current_File_System: ext4
Fstab_File_System: ext4
Backup_Method: files

and

[COLOR="red"]/data | /dev/block/mmcblk0p16[/COLOR] | [COLOR="seagreen"][B][U]Size: 0MB Used: 0MB Free: 0MB Backup Size: 0MB[/U][/B][/COLOR]
Flags: Can_Be_Mounted Can_Be_Wiped Can_Be_Backed_Up Wipe_During_Factory_Reset Wipe_Available_in_GUI IsPresent Has_Data_Media Can_Encrypt_Backup Use_Userdata_Encryption Is_Storage Is_Settings_Storage
Symlink_Path: /data/media
Symlink_Mount_Point: /sdcard
Primary_Block_Device: /dev/block/mmcblk0p16
Display_Name: Data
Storage_Name: Internal Storage
Backup_Path: /data
Backup_Name: data
Backup_Display_Name: Data
Storage_Path: /data/media
Current_File_System: ext4
Fstab_File_System: ext4
Backup_Method: files
MTP_Storage_ID: 65537

I realized this should the a good reason for:

I:Can't probe device /dev/block/mmcblk0p16
I:Unable to mount '/data'
I:Actual block device: '/dev/block/mmcblk0p16', current file system: 'ext4'

I:Can't probe device /dev/block/mmcblk0p14
Failed to mount '/cache' (Invalid argument)
I:Actual block device: '/dev/block/mmcblk0p14', current file system: 'ext4'

5. Therefore, I applied the

mke2fs -T ext4 /dev/block/mmcblk0p8

advice to these other blocks, with

adb shell
mke2fs -T ext4 /dev/block/mmcblk0p14
mke2fs -T ext4 /dev/block/mmcblk0p16

6. In view that both operations were successful, I dared to reboot to recovery. Once TWRP was reloaded (luckily!) I took a look at the recovery script (pressed the icon placed at the bottom right side in TWRP) to discover that there was no 'Unable to mount' issues, except for 'sdcard'. I understood that there was no need to mount, for the recovery would probably have done it for me. It was time for trying bootloader mode and flashing stock firmware via Odin.

7. And it worked! The only remaining odd symptoms are that it takes a couple of minutes for the screen to get active and the "Samsung melody" only sounds partially (big issue, ha?)

I hope this is useful for you although you posted your trouble a few months ago; this happened to me just last weekend.