The honest answer is the biggest and as much as you can reasonably carry. You may never need it, but by Christ you'll be thankful if you've got it when you do... ...and don't forget that different pattern anchors hold better in different seabed materials. Choose one for your local conditions if you can. If you look in the RYA books, I'm pretty sure there is a simple explanation of the different types.
I would suggest something like a 3Kg folding grapnel anchor, 5m of 8mm chain, and 30m of rope.
Folding grapnel anchors aren't the most effective, but they do work and they are easy to store.
Don't skimp on the chain. If you are tempted to go for a bigger anchor, I would just double the length of chain instead!
The anchor rope doesn't have to be massive. Some of the "all in one" anchor kits use 6mm rope, but it's not particularly easy to work with. My preference is for 9mm climbing rope that you can normally pick up cheaply at the Southampton Boat Show.
I got a 4m RIb, and go fishing sometimes, - got a Danforth type folding anchor much better holding propertys than an old grapneI had, this anchor weighs around 4-5 kgs, which should be ample - got 6 metres of warp (chain) and roughly 50 ft of rope, on a depending on the state of tide and where im goin - all depends - jsut the job anyway, small and doesnt take up too much space, let us know how you get on - also I use a quick snap on carabeena on to the front bow eye
I use old 11mm climbing ropes as anchor and shot lines. Usually carry a 50m length in the bow with two anchors. One 2.5kg Bruce and a few metres of chain for lunch stops onto the painter and a 5kg Bruce with 5m of large chain for using in emergency or when I am leaving the boat anchored and going ashore for a while.
Beware using a krab as the sole means of attaching the anchor unless you stay with the boat, they can jump off when the boat is surging back and forth in any chop ( I know, has happened to me and I lost 50m of line, chain and anchor)
Having used Danforth and other pivot types on a small rib I now prefer a Bruce anchor. Holds well in most condition with some chain and has no sharp edges or bits to get fingers nipped in.
i would use a bruce, and 5kg which is easy to carry would be mroe than enough, i used a 5kg on a 4.7m rib, and use it for shorter stays on a 6.3m rib, it holds very well, even with the bigger boat, although id want more if winds picked up, but put with a 4m rib, that boat will be staying put! put bout 4-5m chain with it and ur lauching, bruce anchors are great all rounders, sand stone, u name it....good luck
I've just removed our 7.5kg bruce from our boat this year - because I've never actually used it! I may replace it with a 5kg. I've only ever used our grapnel as it's much easier to recover and stow again.
I reckon that John's got the right idea - keep it small and light, you shouldn't need huge holding power for a 4m RIB, unless you're intending to do some "extreme anchoring".
BTW the best rope is octoplait - it refuses to kink or tangle, and is very soft on the hands whilst recovering. A bit pricey though, and my contact who used to supply it at a very competetive price has ceased trading. For a 4m RIB though, any rope should do the job, and climbing rope has a decent amount of elasticity to absorb shock.