Four rak'ahs before Dhuhr, do they have to be with one tasleem, and is it the same for Duha?

The basic principle is that there is leniency with the voluntary prayers.

There is leniency with the voluntary prayers. It is narrated in the hadith from the Prophet ﷺ: the night prayer is two by two.

And this is in the two Sahihs, and there is another narration that says: the night and day prayer.

Two by two, but the word "day" is not correct, may Allah make it clear; because it is odd, rather it is the night prayer, two by two.

So there is leniency with the voluntary prayers. If someone prays four rak'ahs, for example, with one tasleem, it is allowed, but he should not sit for tashahhud.

He should not make it resemble the Dhuhr prayer; he only sits in the last rak'ah and then gives salam.

But the best is to pray two rak'ahs and give salam. The best is two rak'ahs and give salam, and likewise for Duha, the best is two rak'ahs.

And give salam. If he prays four rak'ahs with one salam, that is permissible.