You can't break 4-rakat faraz or sunnat into two+two. It is not permissible.
Although you can shorten your Salat by leaving Sunnat Ghair-Mo'akadah(Sunnah that are not emphasized). You can't skip Sunnat Mo'akadah(Sunnah that are emphasized) though. Here is a break down.
Fajar
- 2 farz
- 2 Sunnat (Sunnat Mo'akadah)
So 4 Rakat for Salat-al-Fajar is mandatory. No shortening.
Zuhar
- 4 farz
- 4 Sunnat
- 2 Sunnat (Sunnat Mo'akadah)
- 2 Nafal (optional)
So, 10 Rakat for Zuhr is mandatory.
Asar
- 4 farz
- 4 Sunnat (Sunnat Ghair Mo'akadah) (optional)
So, 4 Rakat for Asar is mandatory.
Maghrib
- 3 farz
- 2 sunnat (Sunnat Mo'akadah)
- 2 nafal (optional)
So, 5 rakat for Maghrib is mandatory.
Isha'
- 4 farz
- 4 Sunnat (Sunnat Ghair Mo'akadah) (optional)
- 2 Sunnat (Sunnat Mo'akadah)
- 2 Nafal (optional)
- 3 witar
- 2 Nafal (optional)
So, 9 rakat for Isha' is mandatory.
Other than this, you can't break Salat on your own. I hope this answers your question.