According to Chain of Trust page, X4 will not be in use except there’s something wrong with the current X3 certificate, as it’s for disaster recovery purposes.
Currently, I (personal me) am not sure whether Let’s Encrypt will renew the cross-signed certificate after the IdenTrust crosssign expries… But from what we have now, there’ll be 203 days where X3 is valid but the cross-sign is expired.