The form depends on the sector
A private landlord uses Form 4A. Form 4 is for social landlords. The builder selects on that basis rather than offering both and hoping.
The timing
One increase in any 52-week period, with the notice given far enough ahead that the tenant has time to respond. The builder prints the earliest date the new rent can start, so you are not calculating it from the tenancy start yourself.
If the tenant challenges it
A tenant can refer the proposed rent to the First-tier Tribunal before the increase takes effect. That is their right and it is not a sign anything has gone wrong. The notice, the date it was served and the figures are all in your activity log if you need to show what was proposed and when.
Contractual rent review clauses
Do not rely on one. The statutory route is the route, and a clause in an old agreement does not substitute for a properly served notice.