It is only permissible for travellers to shorten their prayers. If a traveller intends to stay for more than four days he is no longer allowed the concession granted for travellers.
Based on this, you are not allowed to shorten or join your prayers, rather you have to offer the prayers in full, each prayer at its proper time, because you know that you are going to stay in your place of work for 28 days.
It says in Fataawa al-Lajnah al-Daa’imah (8/95):
The traveller who intends to stay in a place for more than four days cannot shorten his prayers. If he is going to stay for less than that, he may shorten his prayers.
Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: What is your opinion about shortening the prayers when travelling a distance that makes shortening the prayers allowable – is that defined by a certain distance? What do you think about one who intends to stay someone for more than four days during his journey – is he allowed the concession of shortening his prayers?
The majority of scholars are of the view that it is defined by the distance of a day and a night’s travel by camel or on foot at a regular pace, which is approximately 80 kilometers, because this distance is customarily regarded as travelling, unlike shorter distances.
The majority also narrated that whoever resolves to stay for more than four days has to offer the prayers in full, and fast if it is Ramadaan. If the period of stay is less than that, he may shorten and join his prayers, and break his fast, because the basic principle is that one who is settled (not travelling) has to offer the prayers in full, and shortening them is only allowed when he starts travelling.
Fataawa Ibn Baaz, 12/270.
The Standing Committee was asked about a man who was far from his family because of his work, as a distance at which shortening the prayers becomes permissible – is it permissible for him to shorten the prayers only whilst travelling between his family and his place of work, noting that the first time he had intended to stay there for a month, for example. They replied:
He may shorten and join his prayers whilst on the road, so long as the distance between his workplace and his home is a distance at which shortening the prayers becomes permissible. If he intended to stay in his place of work for a month, then he is not allowed the concessions of travel in his workplace, rather he should offer every prayer at its proper time, in full.”
Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Ifta’, 8/94, 95
The Standing Committee was also asked (8/109) about a person who traveled to another country for two years: could he shorten his prayers?
They replied: The basic principle is that the traveller is the one who may shorten the four-rak’ah prayers, because Allaah says (Interpretation of the meaning):
“And when you (Muslims) travel in the land, there is no sin on you if you shorten As‑Salaah (the prayer)”
And Ya’la ibn Umayyah said: I said to ‘Umar ibn al-Khattaab (may Allaah be pleased with him): “ ‘there is no sin on you if you shorten As‑Salaah (the prayer) if you fear that the disbelievers may put you in trial (attack you)’ [al-Nisa’ 4:101]; but now the people are safe.” He said: “I wondered the same thing, and I asked the Messenger of Allaah (peace and blessings of Allaah be upon him) about it. He said: ‘This is a favour (lit. charity) that Allaah has granted you, so accept His favour.’” Narrated by Muslim.
The one who is regarded as coming under the ruling on travellers is one who stays for four days and nights or less, because it was proven in the hadeeth of Jaabir and Ibn ‘Abbaas (may Allaah be pleased with them both) that the Prophet (peace and blessings of Allaah be upon him) came to Makkah in the morning of the fourth of Dhu’l-Hijjah for the Farewell Pilgrimage, and he stayed for the fourth, fifth, sixth and seventh, and he prayed Fajr in al-Abtah on the morning of the eighth. He shortened his prayers during these days and he formed the intention to stay there as is well known. So everyone who is travelling and intends to spend the same length of time as the Prophet (peace and blessings of Allaah be upon him) did or less may shorten the prayers. Whoever intends to stay longer than that should offer the prayers in full, because he does not come under the ruling on travellers.