He wants to start fasting on the fourth day

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.