Good news, guys!!
I've just found a way to turn my WinPE4.0 flash drive to a multi-boot WinPE flash drive.
It envloves editing the BCD store, and adding several boot.wim files to the Sources folder on your WinPE4.0 flash drive.
You need to give each WIM file a unique name (without spaces).
Here are the instructions:
NOTES:
E:\ is the letter of the bootable USB flash drive.
bcdedit.exe must be version 6.2.9200 or newer (available by default in Win8).
1. Connect your WinPE flash drive to your Win7/Win8 machine and open CMD with admin rights.
2. To display the existing boot entries in the BCD store, type:
bcdedit /store E:\boot\bcd /ENUM
Among things shown, you'll see the default GUID, which is usually: {7619dcc8-fafe-11d9-b411-000476eba25f}
3. To create a copy of the default entry, type:
bcdedit /store E:\boot\bcd /copy {default} /d "NAME OF ENTRY"
This will add a new entry and will show you the auto-generated GUID it's been given. For example: {19d2ff6e-908f-11e2-acb5-902b343deb0f}
4. To point the default entry to another WIM file, type:
bcdedit /store E:\boot\bcd /set {default} DEVICE ramdisk=[boot]\sources\WinPE40_x64.wim,{GUID of default}
bcdedit /store E:\boot\bcd /set {default} OSDEVICE ramdisk=[boot]\sources\WinPE40_x64.wim,{GUID of default}
*Replace "GUID of default" with the actual GUID.
5. To point the 2nd entry to a WIM file, type:
bcdedit /store E:\boot\bcd /set {newguid} DEVICE ramdisk=[boot]\sources\2nd.wim,{GUID of default}
bcdedit /store E:\boot\bcd /set {newguid} OSDEVICE ramdisk=[boot]\sources\2nd.wim,{GUID of default}
*Replace "GUID of default" with the actual GUID.
You can add more entries by repeating steps 2 and 4.
6. To change the description of an entry:
bcdedit.exe /store E:\Boot\bcd /set {default} description "WinPE4.0 x64"
bcdedit.exe /store E:\Boot\bcd /set {2nd entry GUID} description "WinPE4.0 x86"
bcdedit.exe /store E:\Boot\bcd /set {3rd entry GUID} description "WinRE from Win8 Pro VL x64"
and so on...
*Replace the "XXX entry GUID" with the actual GUID of the entry.
*click on image for larger.
The above info is all good and well for booting from flash drives in non-UEFI mode.
To make the flash drive boot in UEFI mode, we also need to make changes to the bcd file in E:\EFI\Microsoft\boot.
1. First create a copy of the bcd file in E:\EFI\Microsoft\boot just in case something goes wrong (simply copy & paste it, and a new file will be created).
2. Copy your modified "regular" non-UEFI bcd file from E:\boot to E:\EFI\Microsoft\boot (overwrite the existing one).
3. Open CMD with admin rights, and type:
bcdedit /store E:\EFI\Microsoft\boot\bcd /set {default} path "\windows\system32\boot\winload.efi"
bcdedit /store E:\EFI\Microsoft\boot\bcd /set {2nd GUID} path "\windows\system32\boot\winload.efi"
bcdedit /store E:\EFI\Microsoft\boot\bcd /set {3rd GUID} path "\windows\system32\boot\winload.efi"
and so on, if you have more entries (and more WIM files, of course).