It would help to clarify what it's invested in.
(And how was that asset allocation chosen?)
If it's invested in something other than cash then the ARF fund value will presumably fluctuate based on relevant market factors/sentiment and so your 4% monthly payout will also fluctuate?
(Actually, even some "cash" funds may fluctuate slightly in value according to interest rate changes but the fluctuation should be marginal compared to other asset classes where some market determines their value).
If such fluctuation in payments is a problem for you then maybe you need to discuss this with your pension advisor?
As mentioned above an annuity might be one option or maybe drawing a fixed amount rather than a percentage?
On the other hand, at 65 are you also eligible to claim the Benefit Payment for 65 year olds and will you be eligible for the Contributory Old Age Pension from age 66 which would give you another income stream to help buffer such fluctuations?
If you retire at 65, you may qualify for a benefit payment until you reach 66. The payment is similar to Jobseeker’s Benefit, but you don’t need to be looking for work or sign on at your local Intreo centre.
www.citizensinformation.ie
From 1 January 2025, if you apply for the State Pension (Contributory), there is a new way of calculating your rate of payment.
www.citizensinformation.ie
Or maybe you also have other income streams?
In short, if you are drawing 4% from an ARF that is invested in assets that fluctuate in value then you should probably expect the monthly payment amount to also fluctuate.
(Unless pension providers do something like calculate 4% of the ARF value at the start of the year and then pay that pro-rata at 1/12th of that amount each month so that the payment doesn't fluctuate? Or they offer that option?)