Break indices of level 4 (BI 4) correspond to the end of major prosodic units: they mark the boundaries of intonational phrases (IPs).

There are some phenomena which may appear within an intonational phrase (IP) but not accross different IPs.
So, on the one hand, a BI 4 inhibits the application of many sandhi rules such as vowel deletion, vowel coalescence, gliding, and fricative voicing (about the first of these, see Cabré & Prieto 2005 ).

On the other hand, a BI 4 is accompanied by rhythmic cues: it is the domain for pre-boundary lengthening, and it defines the position for pauses.

Finally, it has a precise intonational definition.
The IP is the domain of the minimal tune: it consists of one or more pitch accents plus boundary tones. Thus, a BI 4 is always accompanied by the presence of boundary tones.

On vas? Com hi vas? Quan tornaràs?
[Where are you going? How are you getting there? When will you be back?]

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In this example, there are six prosodic words assembled in three intonational phrases.

The three phrases "on vas?", "com hi vas?" and "quan tornaràs?" are constituted of two PrWords each one, they finish with low boundary tones (L%) and they are followed by a pause.

Thus, these three phrases are three IPs, and must be separated by a break index of level 4.