He wants to start fasting on the fourth day
Question

                Is it permissible to start my sawm on the 4th day of Ramadan?.
Praise be to Allah.
    The fast of Ramadaan is obligatory for every adult, sane 
    Muslim who is able to fast. Whoever meets this description, it is haraam for 
    him not to fast without an excuse, because that is clearly going against the 
    command of Allaah and the command of His Messenger  (peace and blessings 
    of Allaah be upon him), and it is a transgression against the sanctity of 
    this great month.  

    Allaah says (interpretation of the meaning): 

    “O you who believe! Observing As-Sawm (the fasting) is 
    prescribed for you as it was prescribed for those before you, that you may 
    become Al-Muttaqoon (the pious)”

    [al-Baqarah 2:183] 

    “So whoever of you sights (the crescent on the first night 
    of) the month (of Ramadan i.e. is present at his home), he must observe Sawm 
    (fasts) that month, and whoever is ill or on a journey, the same number [of 
    days which one did not observe Sawm (fasts) must be made up] from other 
    days”

    [al-Baqarah 2:185]
Fasting becomes 
    obligatory once it is proven that the month of Ramadaan has begun, by the 
    sighting of the new moon or when the month of Sha’baan is completed with 
    thirty days. 

    The Prophet  (peace and blessings of Allaah be upon him) 
    said: “Islam is built on five (pillars): bearing witness that there is no 
    god but Allaah and that Muhammad is the Messenger of Allaah, establishing 
    regular prayer, paying the zakaah, Hajj, and fasting Ramadaan.” Narrated by 
    al-Bukhaari, 8; Muslim, 16. 

    If your question is about delaying the fast until the fourth 
    day with no excuse, then you know that this is haraam and it is not 
    permissible to do this; rather it is a major sin. See question no. 
    36747. 

    If you are delaying the fast for a reason such as sickness or 
    travel, then there is no sin on you for that, but you have to fast as soon 
    as the excuse no longer applies, whether that is on the fourth day or any 
    other, and you also have to make up the days that you did not fast, because 
    of the verse quoted above in which Allaah says “and whoever is ill or on 
    a journey, the same number [of days which one did not observe Sawm (fasts) 
    must be made up] from other days”, i.e., if a sick person or traveller 
    does not fast, then he has to make up the number of days that he missed when 
    Ramadaan is over.