Question says it all really, how can I run postgres 7.4 on debian lenny? Our developers don't have the resource right now to modify a legacy application we have to work with pg 8.3, and debian etch is no longer supported.
Note that PostgreSQL 7.4 is also not supported. So you might as well go with Etch, since you're going to be on an unsupported version whatever you do.
Your other option is to build from source. PostgreSQL 7.4 should build fine on lenny.