Allaah has permitted men to marry up to four wives, because of the great interests achieved by that, which we cannot explain in detail here. But the matter is not left to the whims and desires of men, rather it is subject to conditions and guidelines so that the purpose of plural marriage may be achieved. 

The problem that you are suffering from stems from neglect of some of these rules and guidelines, namely neglect of the duty to divide one's time fairly among wives. The man is obliged to share his time, night and day, equally among his wives. It is not permissible for him to spend more time with one of them, unless one of them willingly gives up her time as a favour to the other. Otherwise it is not permitted at all.  Allaah says (interpretation of the meaning): 

“and live with them honourably”

[al-Nisa’ 4:19]

Favouring one of the wives does not come under the heading of living with them honourably. There follow some of the comments of the scholars which will explain this further. 

Al-Shaafa’i (may Allaah have mercy on him) said: The Sunnah of the Messenger of Allaah and the view of most of the Muslim scholars indicate that the man must divide his time, night and day, among his wives, and must do so fairly. 

Al-Umm, 5/158. 

And he said: 

I do not know of any differing opinion concerning the fact that a man must share his time equally among his wives. 

Al-Umm, 5/280. 

Al-Baghawi (may Allaah have mercy on him) said: 

If a man has more than one wife, he must share his time among them equally, if they are free woman [i.e., not slaves], whether they are Muslims or women of the people of the Book [Jewish or Christian]… If he fails to share his time among them equally, then he has disobeyed Allaah and has to make up the time for the wife whom he has wronged. It was narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever has two wives and inclines more towards one of them, he will come on the Day of Resurrection with half of his body leaning.” (Narrated by Abu Dawood, 2/242; al-Tirmidhi, 3/447; al-Nasaa’i, 7/64; Ibn Maajah, 1/633. classed as saheeh by Ibn Hajar in Buloogh al-Maraam, 3/310 and by al-Albaani in Irwa’ al-Ghaleel, 7/80). 

What is meant by this inclining has to do with actions; he will not be brought to account for his heart being more inclined (towards one of his wives), if he treats them equally with regard to the division of his time. Allaah says (interpretation of the meaning): 

“You will never be able to do perfect justice between wives even if it is your ardent desire, so do not incline too much to one of them”

[al-Nisa’ 4:129]

 What this means is that you will never be able to do perfect justice as far as what is in the heart is concerned, so do not incline too much towards one of them, i.e., do not follow your whims and desires in your actions. 

Sharh al-Sunnah, 9/150-151. 

Shaykh al-Islam Ibn Taymiyah said:  

He is obliged to treat both wives equally according to the consensus of the Muslims. In the four Sunans it is narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever has two wives…” So he is obliged to divide his time equally. If he stays overnight with one for one or two or three nights, then he must stay overnight with the other for the same amount of time, and he should not favour one of them in the division of his time. 

Majmoo’ al-Fataawa, 32/269. 

Ibn Qudaamah said: 

We do not know of any different opinion among the scholars with regard to the obligation to share one's time equally among co-wives. Allaah says (interpretation of the meaning): 

“and live with them honourably”

[al-Nisa’ 4:19]

And there can be no honourable treatment when one is more inclined towards one of them.

Al-Mughni, 8/138. 

Based on this, what your husband is doing by not dividing his time equally is a great sin in the sight of Allaah. Allaah does not like wrongdoing or oppression, and he does not like those who do wrong. The fact that he thinks it is permissible for him to go on the day allotted to you to his other wife is a mistaken notion.  

Shaykh Mansoor al-Bahooti said: 

“It is haraam for him to enter upon the one whose turn it is not, at night except in case of necessity, and during the day except if there is some need. 

Al-Rawd al-Maraaba’ Sharh Zaad al-Mustanqa’, 6/449. 

His justifying his actions by saying that you do not get along with the first wife is another mistake, and cannot be solved by doing wrong to others. Rather he should adhere to the rulings of sharee’ah in order to solve this matter, and should not do whatever he thinks is appropriate. What is the benefit of this action on his part? Will it solve the problem, or just make matters worse?! 

With regard to your asking him for a divorce, perhaps if you can be patient and put up with him, and also try to reform him, that will be better. You know that divorce is the last resort, because of the many bad consequences that result from it. So you have to offer him a lot of sincere advice and remind him of Allaah, and that this world is transient, and that tomorrow he will meet Allaah, so how will he answer Him about this blatant wrongdoing? 

You can also frighten him with the idea that if this state of affairs continues, you may ask him for a divorce, and perhaps he will understand from that that you cannot put up with this treatment that is not acceptable according to either sharee’ah or reason. You can also seek the help of wise people among your family and ask them to speak to him about that. 

As well as being patient with you, you have to strive hard in making du’aa’ sincerely asking Allaah to guide him and accept his repentance from that. 

With regard to your saying, “it’s not likely that he will change”, this is what you think, but how many wrongdoers and evildoers, and even kaafirs, have been guided and set straight by Allaah, for men's hearts are between two of the fingers of the Most Merciful and He turns them however He wills. 

We ask Allaah to guide your husband and help you both to do that which is good.