The wavelength of a wave is the distance between the two similar features. For example, it can be calculated as the distance between two successive crests or troughs.
If we denote the wavelength of the four case as :`lambda_1, lambda_2, lambda_3 and lambda_4`
and try to figure out the order of wavelength, we will find the following:
Visually, we can see that in case 3, the distance between successive troughs and crests is more than any other case, so case 3 has the highest wavelength. In fact, it has twice the wavelength as compared to case 1 and 2.
Case 1 and 2 have the same wavelength, since the distance between successive crests is same in both cases. So, `lambda_1 = lambda_2`
And case 4 has half the wavelength as case 1 and 2, since for every two successive crest of 1 or 2, case 3 has 2 crests.
Thus in summary: `lambda_4 < lambda_1 = lambda_2 < lambda_3`
and `lambda_3 = 2 lambda_1 = 2 lambda_2 = 4 lambda_4`
Hope this helps