Let 'em live. With 4 dwarves you are lacking a militia and a chamber full of dwarven spawn might slow them down enough to evacuate. Just imagine it like breeding war dogs. Short and drunken war dogs.
You could force them into a training chamber ala dwarven child care or atleast seal them in a separate room until they mature enough to handle a pickaxe.
No, what will slow them down is the hoard of 40 blue peahens ready to release all hell upon anything stupid enough to get in the way (I'm hoping it'll slow them down).
Since I do have that massive breeding population, all will be in full leather (so much easier to suit up 4 than 40), and I'll have enough bone bolts to bring down an entire siege. Just gotta get this construction part of things out of the way. Wall is about 45% complete.
Building materials, wood, clay, and I'll have clay brick roads eventually. The best thing about a small number of dwarves is the rate at which they go through food an drink. Unfortunately since I'm not doing any digging (melted down my copper pick too... Oh, that reminds me, I embarked with tetrahedrite, so I can make them some basic armour each), there's no traps for me... I might use 3 of the 4 remaining marble from embark to construc enough for a retracting bridge to cover the entrance. All caravans have been turned away to this point too...
No fear of necromancers, cause I'm on a small island off the coast (no water anywhere to be seen unfortunately, so I can guarantee that the death of my fort will be due to an injury not being cleaned... Unless unexpected fun happens)
It's actually all looking pretty positive right now... I don't think the lovers have married just yet, but when they finally pop out that kid that's surely on the way, I'll put him to work in the farms... That'll be at least 3 game years away though (2 years to leave mummy, 9 months incubation, and a few months for them to do the deed - yes I know that's backwards)