Parallel Program Execution using Windows Batch File

The requirement is to initiate 4 program executions parallely. The main calling program should not exit until all 4 executions are complete. This is how I did my POC:

a.bat : (Main Calling Script)

start cmd /T:A0 /k "call D:\scratch\sanbox\a1.bat"

start cmd /T:B0 /k "call D:\scratch\sanbox\a2.bat"

start cmd /T:C0 /k "call D:\scratch\sanbox\a3.bat"

start cmd /T:D0 /k "call D:\scratch\sanbox\a4.bat"

:TimePlease

ping 1.1.1.1 -n 3 -w 120000

echo Checking if everything is done

findstr /N ^$ a.out | findstr 4

if NOT %ERRORLEVEL% == 0 goto TimePlease

The 4 programs that were to be executed parallely:

a1.bat :

@echo "In A1 Execution"

ping 1.1.1.1 -n 10 -w 7000

@echo "A Done" >> a.out

a2.bat :

@echo "In A2 Execution"

ping 1.1.1.1 -n 30 -w 1000

@echo "AA Done" >> a.out

a3.bat :

@echo "In A3 Execution"

ping 1.1.1.1 -n 70 -w 3000

@echo "AAA Done" >> a.out

a4.bat :

@echo "In A4 Execution"

ping 1.1.1.1 -n 50 -w 5000

@echo "AAAA Done" >> a.out

The way it works is, when I invoke master script a.bat, it invokes 4 programs, named a1,a2,a3,a4. All these programs at the end of their execution, create an entry into common file a.out. Master script keeps polling a.out until it finds 4 lines in it.