I don't know which pollsters they are going to use. You can find all the recent requirements and rules that changed here (and the explanation why they changed them):

https://mma.prnewswire.com/media/1538034/Leaders__Debates_Commission_Leaders__Debates_Commissioner_David.pdf

This is what they say about the 4% requirement:

(iii): five days after the date the general election is called, the party receives a level of national support of at least 4%, determined by voting intention, and as measured by leading national public opinion polling organizations, using the average of those organizations’ most recently publicly-reported results.

With regards to criteria (iii), the Commission will select public opinion polls based on the quality of the methodology employed, the reputation of the polling organizations, and the frequency and timeliness of the polling conducted. The Commission may take professional advice to assist in selecting the leading national public opinion polls to be used in applying the criterion and will identify the selected polling organizations at the time the criterion is applied.