Based on my recent experience with Budgie Babies, 3 seems to be the maximum that Budgies can feed really well. And Mama Budgie was spending all her time and energy collecting food & feeding the babies. So, I would Not recommend attempting to add more to the Budgie Parents' responsibilities, I think you would likely lose the existing baby budgies instead.
Also. You are concerned because the IRN parents did not feed the first baby and it died. It is Possible that First Baby had something wrong with it. It seems odd to me that both parents would choose to not feed it, otherwise.
IF the second one hatches and also does not get fed, you could try doing what I did with my Last Budgie Baby. Mama could not keep track of them all, after three. Number Four, Tiny, had not grown any and was about to die. I hand-fed her but did Not pull her from the nest otherwise. So I would take her out and feed her, but then put her back. Day Three, Tiny had gotten a little bigger (barely!), enough that Mama found her and began stuffing her with food.
So IF your next IRN baby is not getting fed, you could try this as well. Rather than KEEP the baby Out, instead, ONLY take it out for some extra feedings, IF it seems the parents also ignore this one. Then put it back each time for them to keep the baby warm, etc. There's a reasonable chance they might then realize they have a baby to feed, and take over the job.
Again, I doubt whether Budgie-mama can come up with sufficient food to nourish a relatively Huge IRN baby, but if so, it seems it would likely be at expense of her own babies.