It's the standard free-market residential lease: 4 years renewable for another 4, at a freely negotiated rent. It's the default contract type for long-term housing in most Italian cities.
It gives strong tenant protection: the landlord can't easily refuse the second 4-year renewal without specific reasons. The rent can be adjusted yearly by ISTAT unless the cedolare secca was chosen.
"After a few months house-hunting in Trento, Iulia signed a 4+4 lease in a quiet neighbourhood. The landlord opted for cedolare secca at 21%, and her rent was locked in for the duration."
Tenants can leave with 6 months' notice (or less if the contract allows) for grave reasons. Landlords have a much harder time getting tenants out within the 4-year periods.
Official source
Agenzia delle Entrate
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