If the quaternary consumer is eliminated from the food web, the most likely effect would be that the populations of its primary prey (the tertiary consumers) would increase. This is because there would be less predation pressure on them, allowing their numbers to grow. Consequently, if the tertiary consumers (such as snakes and frogs) are increasing, the populations of their prey (which could include grasshoppers, squirrels, etc.) would likely decrease due to increased predation.
As a result, it can lead to a situation where the producer population (plants and other autotrophs) may also decrease due to overgrazing or overbrowsing by the increased herbivore population (the grasshoppers and squirrels in this case).
Therefore, the most accurate statement would be:
The grasshopper and squirrel population would increase.
However, this may lead to a decline in producer populations if those herbivore populations grow excessively.