Although the 4th dose is yet to be confirmed. This is the info from Blood Cancer UK:
Third vaccine dose for immunosuppressed people
Adults and children aged 12 and over who are severely immunosuppressed should get a third dose of the covid vaccine this autumn. The advice covers the whole of the UK.
This is different from the national booster programme. If you have blood cancer, it's important to get your third dose, rather than the booster. This is because for some types of vaccine (like Moderna), the third dose is a full dose but the booster dose will only be a half dose. It's also important because it gives you the chance to have three primary doses rather than two, and possibly a fourth dose (booster) in the future as well (although we are waiting for confirmation on future booster doses for this group). Having a third dose aims to increase your antibody response to the vaccine.
The third dose is for people who had weakened immune systems at the time of their first or second vaccination, who might not have produced enough antibodies from the vaccine. This includes anyone with blood cancer or ongoing immunosuppression. The third dose aims to increase the level of protection gained from the vaccine.
The third dose is different from the booster jab, which other vulnerable groups (who aren't immunosuppressed) are being offered, and which only aims to extend the length of time that the original protection lasts.
People who get a third dose of the vaccine this autumn may also get a booster vaccination in around six months' time, but this hasn't been confirmed yet.