To answer the question - yes the CGT can be paid from the sale proceeds held.
Going a little further under the hood, I wonder is this actually a sale by an executor or administrator, with sale proceeds to be then distributed to four beneficiaries (all Irish resident) ?
If so, the CGT liability is that of the estate, and the executor or administrator is liable for making the return and paying the tax.
Is this the situation? And is the executor one of the four children?
If so, in this situation, the solicitor has no particular liability for this CGT. However, the solicitor does have a duty to protect the client executor.
If all four beneficiaries are happy to sort this out among themselves, they can agree this with the executor and the executor can just tell the solicitor to distribute funds. (To be clear, if the facts are as surmised by me, what the solicitor is seeking to avoid is the executor being on the hook for CGT but not having funds kept back for it, and asking the solicitor 'how did you let this happen?')
Do we have to wait until CGT is completed before the remainder of the estate can be equally distributed to the siblings (÷) 4 ?
Even a very prudent executor (if I have correctly understood the situation) should be happy to distribute the bulk of the money now, holding back enough to cover maximum likely CGT and filing costs. Chances are that the CGT is already ascertained to a high degree of certainty anyway.