4bytes ASn and RFC1745

what are the consequences of 4bytes ASn (RFC4893) for RFC1745 -
BGP4/IDRP for IP---OSPF Interaction?

In particular with regards to storing the AutonomousSystem in the lowest
16 bits of the External Route Tag (=32 bits)?

Little practical consequences I expect. This is taken from RFC3167, section 1, in 2001:

During a review of internet standards relating to BGP, it became apparent that BGP/IDRP OSPF interaction, as described in RFC 1745, has never been deployed in the public internet, and would require significant implementation complexity. Since this mechanism has never been in use in the public internet, it is proposed to reclassify it to Historic.