As'salamu Alaikum I tied ihram for hajj at the age of 4 years but all my arqams(rituals) were done by my father. So, as i am 20 years old now is it fard for me to do that hajj again.
In Islam children are neither asked nor ordered to perform hajj nor to perform any worship until they reache puberty as their actions won't be recorded as stated in the known hadith:
There are three whose actions are not recorded: a lunatic whose mind is deranged till he is restored to consciousness, a sleeper till he awakes, and a boy till he reaches puberty?
(Sunan abi Dawod, sunan an-Nasa-i and sunan ibn Majah)
But even in the early days of Islam it was usual to have children in the company when performing Hajj and other worships as this hadith shows:
“We performed Hajj with the Messenger of Allah (ﷺ), and there were women and children with us. We recited Talbiyah on behalf of the children and stoned the Pillars on their behalf.”
(Sunan ibn Majah)
This is a way to teach them and let theme learn about Islam from an early age on.
However if they do a worship their parents would be rewarded for teaching them as this hadith shows (and they would also get the rewards for that worship, but it doesn't count as having fulfilled an obligatory worship):
A woman lifted up her child and said: Messenger of Allah, would the child be credited with having performed the Hajj?
Thereupon he said: Yes, and there would be a reward for you.
(See in sahih Muslim on the authority of ibn 'Abbas and Kuraib -mursal-, it was also compiled on the authority of ibn 'Abbas in the sunan of an-Nasa-i -who narrated a version of Kuraib from ibn 'Abbas, which should be that of al-Muwatta'-, in sunan abi Dawod and al-Muwatta' and on the authority of Jabir in sunan ibn Majah and Jami' at-Tirmidhi)
Scholars interpreted this hadith as follows:
Al-Nawawi said in Sharh Muslim:
This is used as evidence by al-Shaafa’i, Maalik, Ahmad and the majority of scholars that the Hajj of a child counts and is valid, and he will be rewarded for it, but it does not count as the Hajj of Islam (the obligatory Hajj), rather it is regarded as a voluntary Hajj. This hadeeth clearly states that.
Al-Qaadi said: They are unanimously agreed that it does not count as the obligatory Hajj, which he must do after he reaches puberty, except for a small group who held an odd view and said that it does count. But the scholars did not pay any attention to this view.
The words of the Prophet (peace and blessings of Allaah be upon him) “and you will have the reward” mean that this is because of her carrying him and making him avoid the things that are forbidden to the pilgrim in ihraam, and do what the pilgrim does. End quote. (source: islamqa #85299)
And in the commentary of sunan abi Dawod:
It is Hajj for him in the sense of reward, without being counted as Hajj in the sense of obligatory Hajj if he lives until he reaches puberty and becomes a man. This is like prayer; he should be told to pray when he becomes able to pray, although it is not obligatory for him, and the reward will be written for him by the bounty of Allaah, and also for the one who tells him to pray and teaches him to do so. If he does Hajj then he must do the all the rituals, such as standing in ‘Arafah and circumambulating around the Ka’bah, being carried if he is unable to walk, and doing saa’i between al-Safa and al-Marwah, and other actions of Hajj. End quote from ‘Awn al-Ma’bood. (source: islamqa #85299)
Ibn Qudaamah said in al-Mughni, 5/44: Ibn al-Mundhir said: The scholars are agreed – apart from those who held odd views and whose opinions are not significant – that if a child does Hajj when he is small, and if a slave does Hajj when he is still enslaved, then the child reaches adolescence and the slave gains his freedom, they have to do the obligatory Hajj, if they have the means to do so. (source: islamqa #36862)
So the hajj which Allah has asked every Muslim who is able to perform it (or posses the means to do it) is still a due on you.
There's even a hadith on the authority of ibn 'Abbas compiled by ibn Shayba's and imam a-Shafi'i in his musnad and qualified as sahih by Sheikh al-Albani saying:
“Any child who does Hajj then reaches puberty, he must do Hajj again.”