hiya a nero 4 should be fine for 2 piggies, ive got mine in a nero 3 deluxe and they seem happy enough

and thats smaller than the nero 4
for housing there are a number of flooring you can choose from for the piggies, i personally have wood shavings for my girls outside (soon to change as i dont like shavings anymore) i have pro-fleece for lois as i need to keep her clean, and my 2 boys are on newspaper and hay, newspaper and hay is the cheapest option, and the easiest i dont like my pro-fleece anymore it takes ages to clean and brush all the muck up off etc... when i run out of newspaper and hay for the boys (i only use cheap hay for bedding and i buy more expensive stuff for them to eat) I use towels and 100% polyester sheet (voile), layer about 5 towels in he bottom of the cage and place the polyester sheet over the top weight it down in each corner, place the top wire cage bit on and just peg it on around the cage, and take the weights out, its so easy to sweep hay and poop off daily unlike fleece which everything sticks to - have a look in the housing section for lots of diff bedding ideas.
There are lots of different food on the marker too, which most of us have tried a few of, mine are on science selective pellets atm - id deffo go for a pellet food rather than a muesli one as it doesnt cause them to pick and choose which bits they eat therefore they are getting everything that they need from the food

hay is a big factor in the diet, although your bunnies probably eat plenty of that anyway.. unlimited hay is recommended, help keep the teeth down and the digestive system in good working condition, they need fresh veg and the occasional fruit treat as they can't make their own vit C so they get this from the fresh foods.. if you have a look in the food section there is a shopping list and also reviews on different dried foods
Caring for a guinea pig is fairly easy as far as keeping them clean, fresh veg etc making sure they get the correct diet, are not too hot/cold, no where that it is drafty, not in a garage where fumes are, they cannot be kept anywhere around those plug in air freshners and its best not to spray air freshner in the same room as them, and as suggested above a good vet is always a plus!
If you get a long haired guinea pig then you need to allow time for brushing, and grooming on a daily basis only takes 5 mins to comb through the hair, and it needs a hair cut every few weeks if your keeping the fur short so as not become grubby, and possibly a bath if they get really grubby.
www.pamperedpiggies.co.uk <-- Good for toys, treats, bedding and general piggy stuff
www.gorgeousguineas.com <--- Good for bath time shampoo, they also have natural treatments for lice/other nasties
hope that helps
