I got married and my wife did not get pregnant for almost a year. I made a vow to slaughter a sheep if Allah, the Exalted, would bless me with a boy. When Allah actually blessed me with a boy, I said, "I will slaughter another sheep because the baby is a boy." This means that I need to slaughter four sheep. Can I slaughter a cow instead of the four sheep? Its price is equivalent to that of the four sheep? Is it permissible for me to eat from it, or should I offer it all in charity? Can I offer a male goat instead of the sheep?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Vows are not encouraged under the Islamic Sharee'ah, based on the Hadeeth in which the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, forbade Muslims from making vows, as he said: "A vow does not repel anything (which is predestined), but it is a means whereby (something) is extracted from the miserly person." [Al-Bukhari and Muslim]

However, it is incumbent on whoever makes a vow to do an act of obedience to fulfill it given that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to do an act of obedience to Allaah, the Exalted, must perform it, and whoever vowed to disobey Allaah, the Exalted, should not do so." [Al-Bukhari]

Hence, you vowed to slaughter a ewe if you are blessed with a baby boy, and you have been blessed with one, all praise is due to Allaah. Then, you  voluntarily added another ewe.

As for the first ewe, the rulings pertinent to vows apply to it. This means that it is incumbent on you to deliver on that vow and slaughter a ewe. The Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to do an act of obedience to Allaah, the Exalted, must perform it, and whoever vowed to disobey Allaah, the Exalted, should not do so." [Al-Bukhari]

Slaughtering a cow or a male goat or anything else will not be a valid fulfillment of your vow. This is because you made a vow to perform a particular act of obedience for Allaah, the Exalted, and it is obligatory on you to do exactly what you vowed to do. Eating from the vowed ewe is permissible unless you intended to give all of it to the poor only. If your intention was to offer it all to the poor, then it is impermissible for you to eat from it or avail yourself of it in any manner because, in this case, it is exclusively dedicated to them. Ibn Abi Shaybah reported the following on the authority of ‘Ata’ :

"When the animal is slaughtered as an expiation for violating the state of Ihraam (state of ritual consecration) during the Hajj journey by hunting, for example, or when one vows to slaughter an animal and offer all of it to the poor, then it is impermissible for the vow-maker to eat from the animal in both cases."  [Al-Musannaf]

The second ewe is offered as a voluntary act of worship; you are entitled to dispose of it in whatever way you wish. 

The other two ewes are for the ‘Aqeeqah.

Allaah Knows best.