If you keep 4 males & 4 females together then the males will over-harrass the females and this stress can make them vulnerable to fish diseases like finrot & columnaris (which can prove lethal to fish). So if your dad is serious about keeping fish, then it is better for the whole tank and every fish in it to keep proper stocking levels/ratio's.
Guppys are the easiest fish to breed out there, if you keep 1male per 2-3females you will have loads of fry in no time- it is not necessary at all to keep a high level males to female ratio, a healthy male guppy will spend all day every day mating or trying to mate with the females.
Your dad is being greedy/selfish but there is actually no solid rationale behind keeping a higher number of males as the females may actually produce less fry out of stress if they feel the environment is not good (female guppys have the ability to re-absorb or abort pregnancies if the female is under a great deal of stress). Guppys can also eat their or each others fry and so the less males the tank has the less likely the fry are to get eaten.
If you are definite on getting a mixed gender group then make sure you have the space to raise the fry. A female guppy can on average gives birth to 25-30 fry a month and these fry will take at least 3-6 months before they are of a sellable size (though given how numerous guppys are you will probably have to rehome them for free) and so having a decent sized fry raising tank (ideally 10gallons+ which has good filtration & heating) is a must
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If you are not prepared to raise every fry which gets born in the tank, you must face the prospect of culling them (humanely). While most fish are partial to eating fry, some fish are more so than others and so some fry will almost always survive whether you want them to or not.
