SkyFactory 3 balooned to 14 Gig, because of my extensive use of Chisels n Bits, but yeah, you want to have a tad more than 5 gig avaible solely for minecraft.Your computer is a little low on RAM to handle much in the line of mods. (And I've been there with the kid-schedule.) Try out the vanilla game first off, and see if you like it. There's still tons of stuff you can do. If you like it and want to get into mods, you'll want to add a bit more RAM - I have 8 Gb, and haven't had much trouble with most of the 1.7 modpacks.