How we cite our quotes:
Quote #4
HILDY: This is the biggest thing in my life.
Hildy's referring to the story she's writing, and to her career. Bruce is upset to hear that she's putting her career ahead of him. Bruce is kind of a jerk (though, to be fair, he's had a rough day.)
Quote #5
HILDY: How you have messed up my life. What am I going to do?... I could be on that train right now. What a sap I am, falling for your line: "They're gonna name streets after me." Johnson Street!
"What a sap I am, falling for your line" sounds like it means she's fallen for a romantic line. Actually she fell for Walter telling her how famous she'd be. But the two things are constantly tangled together in His Girl Friday. Walter is Hildy's ambition personified; he's both the career and the guy who wants to marry her. It's hard to separate the two.