My ISP provides a main email account with a login email address and password. I am allowed another 4 email addresses, each with a different password which I can access directly through webmail once I have logged into the main account. But i would like to configure Pegasus to collect email from each of the other four addresses so that i can use a different identity for each that has a different sender address. Can this be done, and if so, can someone please guide me as to how ?

Yes, however your post is a little confusing. You used the term "IMAP' in the subject line but then said "configure Pegasus to collect email from each of the other four addresses" which indicates to me the downloading of mail via POP3. When using IMAP, each email address would need its own IMAP profile. When using POP3, each account will need its own identity. If POP3, read my post in this discussion for more information about identities.
https://community.pmail.com/index.php?u=/topic/11893/really-screwed-up-identities-can-t-send-e-mail-smtp

Multiple identities are valuable for what you are doing but only for POP3. They add a level of complexity the requires of good understanding of them. Reading the sections of the manual that relate to Identities will help. Filters that direct mail to identity specific folders helps with keeping the mail sorted. Associating the appropriate identity to those folders will automates the "From" information when doing replies.

If any email accounts belong to different people then additional Users may be more appropriate than Identities.