To me that doesn’t sound like enough insulin, BUT everyone is different and it might be that you don’t need so much.
How did you set the app up? Most people start on a ratio of 1:10 when carb counting, which for 126g carbs would mean 12.6 units, probably rounded up to 13. So if it’s only suggesting 4 then it thinks you are on a ratio of 1:30 or something which would be unusual. Although not impossible!
If you have been having 8 units for lunch previously, have you been eating similar amounts of food and what happened to your blood sugars afterwards? If you ate about the same amount of carbs and injected 8 units and didn’t have a massive hypo afterwards then 4 units isn’t going to be enough.
The app will also take into account what your blood sugars are now and will add or subtract a bit from the bolus to bring them back into line if necessary, you need to set all that information up correctly too. If you don’t know how to do that then you need to ask your diabetes team for advice.