Based on the GCP document Free Tier usage limits
App Engine has 28 hours per day of "F" Instancees.
I wonder does F4_1G type also belong to the free tier of 28 hours per day?
F1 and F4_1G is so different Instance classes
To answer your question, yes F4_1G is also under the free tier offered by Google Cloud Platform. The only difference of the F4_1G to the F1 is in terms of the amount of memory and CPU available to each instance, hence the cost. Here's an example:
F1 would cost $0.05 per hour per instance, while the F4_1G would cost $0.30 (based on the Iowa (us-central1) location). Check this pricing list to know the cost per region.
Therefore, when using F4_1G, you're using x6 of the usual default instance hours. In conclusion, you'll use "24 instance hours" if you run an F4_1G non-stop for 4 hours.