it really depends on what you are fishing for. yes, 4 goes with 4 and 5 goes with 5, but if you need to you can go up or down 1 number. larger fish, heavier leaders. go to www.christopherflaveflyfishing.com and see what he has. best leaders there are and you can contact him with any questions. tell him i sent you!

As DCulver said, the leader diameter and line weight are not related.

You will need to adjust your tippet / leader size to the fishing you're doing on any given day. Smaller tippet (higher number) is good for bright, clear conditions and spooky fish, or for more delicate presentation of smaller flies. Of course, it also has lower breaking strength. Larger tippet, conversely, will turn over a larger, heavier fly, and will withstand more abuse.

Length is also determined by how you plan to fish. Longer leaders can make softer presentations, and keep the line further from a fish in clear conditions. They can also get you into deeper water when combined with weighted flies while using a floating line. But the fly is carried by the weight of the line, not the leader, so the longer your leader, the harder it is to cast a heavier fly or a long distance (particularly if you're fighting the wind).

Different leaders can make a huge difference in how many fish you catch in some places. When fishing areas that see a lot of pressure, where fish are educated, and in clear water or bright conditions, smaller diamter, longer, lighter leaders are the rule. When fishing murky water or less cautious fish, it doesn't matter as much.

I typically fish 5x or 6x, 9 to 12 foot leaders for trout in very clear water.

So no one understands what leader/tippet diameter 4X tippet is, or any other size tippet? No one knows the formula, and how it matches what flies you choose to use, or how that matches up to the rod/line size you use? My word!

With respect sir, the question indicates you are very new to fly fishing. As stated the size of a tippet is completely different from the wt. of a line. Tippet size and leader building are a question of extreme debate. There are, however, some very general considerations. Primarily what will you be fishing with (i.e., big ol' 2/0 bass bug or itty bitty #28 midge) and concerns regarding spooking fish demanding a longer tippet. The Big Ol' Bass Bag on a 2/0 hook can be used with 5 1/2 to 6 feet of 10-15 lb monofilament without worrying to much about the line diameter, the midge in crystal clear water to fish that spook when wind is broke might need a very small diameter 14 ft leader. Given the tremendous variance in tippet construction I suggest hooking up with a local fly fishing club, or attending one of Orvis' free clinics and get a knowledgeable fly fisherman to help you.