Fourth Suit Suggestions

What context will you be wearing the suit?

Business?

... depends on your local climate. If you live somewhere perpetually warm, a summer suit would be a good next choice. Linen or fresco-style light wool, in a lighter colour. That might be a good choice generally as it's easier to wear an extra t-shrit or long undies to make a summer suit work for the fall, but a thick suit can't work in the heat no matter what.

Grey

FTFY and +1

or light grey

I'd go light grey as well. More versatile than pinstripes.

Stripes are a no-go for weddings, engagements, and parties. If you want something heavier how about a grey flannel in a subtle plaid or windowpane?

Medium or light grey

Light or mid grey, but with some kind of subtle PoW check.

Tan/Khaki lightweight wool, maybe in a subtle herringbone.

Mid-grey or RAF blue flannel around 13oz. Mid-grey could have a very subtle windowpane. Or, if you're feeling slightly more daring, a nice, subdued flannel PoW (predominantly grey or tan, I'd say). Or you could do a blazersuit in a blue hopsack of sorts.

You have the basics covered. If you want a navy chalkstripe, go for it; but just know that it is a work suit. You have 3 others you can wear to the weddings, parties, etc.