What is the frequency of pure round seeded pea plants in ${\mathrm{F}}_2$ generation of dihybrid cross?

In second filial generation in a dihybrid cross, 

Pure round seed pea plants will have genotype RRYY, RRYy, RRyy. 
Probability of getting RRYY  out of 16 possible combinations is 1.
Probability of getting RRYy out of 16 possible combinations is 2.
Probability of getting RRyy out of 16 possible combinations is 1.
Therefore, the total probability of getting pure round seeded pea plants in ${\mathrm{F}}_2$ generation, out of 16 possible combinations is 4.