Part-time work after baby — how to negotiate a reduced arrangement

Currently on maternity leave and dreading going back full-time. My baby will only be 4 months and I really want to negotiate a 4-day week at least for the first year. Has anyone successfully negotiated part-time or flexible arrangements in Singapore? What's the conversation like with HR? Do you frame it as temporary or ask for a permanent change? I'm worried about being sidelined for promotions if I go part-time but the thought of 5 full days away from her is breaking my heart.