the best trick to get a "large orchestra" sound from a quartet is to double it once, maybe twice but then double it a couple of rounds played with sordino. Maybe even miking the sordino takes from a distance, depends on your room.
Also try to double, again normal + sordino, without the cello. You can get too much cello with many doubles.
If they play well, doubling with samples should work fine. I've done it with good results.
there's a point where more doubles makes it worse, it just doesn't get bigger only "squeekier". My guess would be two normal takes, one or two with sordino, one or two without cello (with or without sordino) and then some samples blended in. That should give you a big sound.
For miking, try an AB or XY stereo with your 414's or 88's or MS with U47+royer or whatever works. The U47 is great for spot miking the cello aswell if you need to do that (probably not). You should be ok with those mics.
Good luck, pop strings are fun.