I'd like some ideas on would be an ideal build for a Repulsive-class Grand Cruiser. my group wants to invest more in its construction but don't want to downgrade the crew to incompetent.

I only have 4 ship points without the downgrade.

any advice? should I downgrade or make do with 4?

With just 4 you won't be able even to get proper macro-batteries for it.

I would downgrade (as it's not so hard to get this fixed later) and use poor craftsmanship components for now.

(as you should have plenty of space and power in that beauty to use)

Also, you can argue with your GM if you can get a "Finance in Arrears" in addition to any other oddities you may already have for that ship, and then get extra +5SP or +10SP for it in addition to a special clause in your warrant (e.g.: the Imperial Navy gave you this reserve ship on the condition you will have to liberate an specific colony from Chaos occupation before the end of the year).

Let us know how much SP you will have in the end and what focus you'd like for that ship (Exploration, carrier, invasion, long range, resilience) and we will put together some template suggestions. ; )

Sure four will leave it a little under gunned but my plan at least is to add more firepower over the course of the campaign.