How to extract system.img.ext4_* files? (Windows)

Hello,

so I extracted a .DZ ROM file. The system.img(.ext4) has 6 parts:

system.img.ext4_0
...
system.img.ext4_5

How can I merge them into a functional system.img(.ext4) file in Windows?

I've used this cmd command:

copy /b "system.img.ext4_*" "system.img"

Then I used unyaffs to extract it, but it returned this errors:

7 [main] unyaffs 1564 _cygtls::handle_exceptions: Exception: STATUS_ACCESS_VIOLATION
1529 [main] unyaffs 1564 open_stackdumpfile: Dumping stack trace to unyaffs.exe.stackdump
1114863 [main] unyaffs 1564 _cygtls::handle_exceptions: Exception: STATUS_ACCESS_VIOLATION
1131084 [main] unyaffs 1564 _cygtls::handle_exceptions: Error while dumping state (probably corrupted stack)

The content of unyaffs.exe.stackdump is:

Exception: STATUS_ACCESS_VIOLATION at eip=610DEFA1
eax=00000000 ebx=00000009 ecx=FFFFFFFF edx=00000000 esi=611021A0 edi=00000000
ebp=0028CC08 esp=0028CC04 program=C:\***\unyaffs.exe, pid 1564, thread main
cs=0023 ds=002B es=002B fs=0053 gs=002B ss=002B
Stack trace:
Frame Function Args
0028CC08 610DEFA1 (00000000, 0040DC70, 00000200, 610E5635)
0028CC48 004010BF (0028CCA9, 00000000, 0040130E, 00401333)
0028CC78 004013AF (00000002, 00F401B0, 00F40090, 610BEE77)
0028CD58 610060D8 (00000000, 0028CD90, 61005450, 0028CD90)
61005450 61004416 (0000009C, A02404C7, E8611021, FFFFFF48)
1114863 [main] unyaffs 1564 _cygtls::handle_exceptions: Exception: STATUS_ACCESS_VIOLATION
1131084 [main] unyaffs 1564 _cygtls::handle_exceptions: Error while dumping state (probably corrupted stack)

What am I doing wrong?