The key thing to develop your own strategy and decide how wide you want to go is to understand the happiness system. The tradition 4 cities is a solid and famous strategy, but you should also learn to decide by yourself based on map conditions. CiV has a huge quantity of variables to consider when making decisions such as when/where/how many cities to build, that is what makes it a great game and makes humans so overpower compared to AIs
The happiness system is divided in local happiness and global happiness. Local happiness is the happiness generated by buildings such as Colosseum and Circus and the religious buildings (like Pagodas), and it is only used to counter your "unhappiness by number of citizens". This means that if your city has 1 citizen and you buy a Colosseum, this Colosseum will generate only 1 happiness, not 2. It will only generate as much happiness as the number of citizens, because local happiness, as the name says, is divided by each city.
Now, there is also global happiness, which is generated by luxury resources, Mercantile City-States and Natural/National/World Wonders. Global happiness you get is yours right away, with no other requirements. This is the happiness you want to counter your "unhappiness by number of cities".
Considering that happiness is your real limit of population (and population = science), minimizing your unhappiness by number of cities and maximizing food growth in your cities naturally makes you have a larger population before you reach the happiness limit; that is why the first answer to your question is that a tall empire has better science.
But keep in mind other factors: the first one, that you should consider as soon as you scout the map in the first turns, is luxury resources. Each different luxury is 4 global happiness, and each new city is 3 unhappiness for number of cities. This means that if making a new city will give you a luxury resource that you would not be able to get in any other way, settling this city is almost always profitable (just make sure that it will have production to make stuff, food to grow and is in a location that you can defend from attacks). Another factor is that there is a limit to how much local happiness you can have per city. A one-city empire with a huge population and a Colosseum may have a lower happiness limit (and, consequently, a smaller total population) than a wide empire with Colosseum, Pagoda and Mosque in every city, plus Circus in about half of them, given they have enough population to actually generate this local happiness. This means that a wide empire should never keep cities small, because you want more population to use the local happiness, and obviously to have more science/gold/production. Also note that religion is a strong source of happiness (mainly local happiness, now that Cerimonal Burials was nerfed), and wide empires are notoriously better at generating faith.
Finally, consider the local happiness generated by social policies. Both Liberty and Tradition have happiness-generating policies: from Tradition, you get a huge amount of local happiness but only in your capital, while from Liberty you get 1 happiness from every city, but it depends on you getting roads connecting all of them, and roads cost gold.
All of this comes down to: different empire sizes are better suited to different map conditions, and knowing the system will allow you to choose the better one.