Stamp 4 is an immigration permission to stay in Ireland for a specified period, subject to conditions. This stamp is issued for long term residence to several categories for long term residence

All applicants and family members who have been approved under Irish immigrant investor program will receive a residence permit card with stamp 4.

Ireland since Dec 2017 has replaced GNIB cards with new format of residence cards

Stamp 4 Permission

Stamp 4 permission allows you to

Take up employment and are not required to hold an Employment Permit.
You can work in a profession, subject to conditions of the relevant professional or other bodies.
You can establish and operate a business.
You may access state funds and services as determined by Government departments or agencies.
Apply to renew your permission and registration before they expire.
Apply for naturalization after five years

Stamps

The Stamps indicate several immigration categories subjected to conditions

Stamp 0 indicates permission to stay in Ireland for a temporary period
Stamp 1 indicates permission to work or operate a business in Ireland, subject to conditions.
Stamp 1A indicates permission in full time, paid accountancy training (with a named organisation such as CPA Ireland, ICAI or regulated by the IAASA and with a training contract with a company based in Ireland) for a specified period
Stamp 1G indicates you have finished your studies in Ireland and have permission to look for employment here under the Third Level Graduate Programme,
Stamp 2 indicates permission to study a full time course on the official Interim List of Eligible Programmes (ILEP)for a specified period
Stamp 2A indicates permission for full time study in Ireland for a course that is not on the official Interim List of Eligible Programmes (ILEP)
Stamp 3 indicates permission to stay in Ireland for a specified period, subject to conditions.
Stamp 4 indicates permission to stay in Ireland for a specified period, subject to conditions.
Stamp 5 indicates permission to stay in Ireland without limits on the time you can remain in Ireland
Stamp 6 indicates you are an Irish citizen with dual-citizenship. Stamp 6 issued in your non-Irish passport if you have applied for permission to Remain in Ireland ‘Without condition’