As with all Grand Theft Auto games, GTA 4 promises a variety of weapons. The list below is based solely on screen shots and videos of the game thus far and will be updated with the full list, once the game is released.

PSG-1 - Sniper Rifle
Capacity: 20, 7.62x51mm NATO

The PSG-1 is mechanically based on the G3 rifle and features a low-noise bolt closing device. Its expected accuracy is below 1 minute of arc (MOA) and it is considered to be one of the most accurate semi-automatic sniper rifles of its caliber, capable of putting 50 rounds of match ammunition inside an 80 mm circle at 300 meters.

Knife
Capacity: 1

A knife is a handheld sharp-edged instrument consisting of handle attached to a blade used for cutting. The knife is a tool that can be used as a weapon. Its origins date as far back as two-and-a-half million years ago, as evidenced by the Oldowan tools.

Micro Uzi - SMG
Capacity: 20, 9x19mm

The Uzi is a family of guns that started with a compact, boxy, and lightweight submachine gun. Smaller and newer variants are considered machine pistols. The first Uzi submachine gun was designed by Uziel Gal in the late 1940s. It was manufactured by Israel Military Industries, FN Herstal, and others.

SW MP-10 - SMG
Capacity: 30, 9x19mm

The MP10 fires from a closed and locked bolt in semi-automatic and full automatic mode. This gun has a unique roller delayed blowback - locked-bolt system, full-stock that folds to the side for transport for shooting, and can be fitted with an illuminating flashlight integral to the forward end of the receiver. This is a improved version of the Hk MP5 for the next century. Source

Winchester 1200 - Shotgun
Capacity: 7, 12/16/20 Gauge

The Winchester Model 1200 is a US made pump action, repeating shotgun. The 1200 Defender model has a 7 shell capacity magazine tube, while the hunting variants have smaller capacities to comply with US hunting regulations. The Model 1200 was produced in 20, 16 and 12 gauge.

M40 - Sniper Rifle
Capacity: 5, 7.62 x 51mm NATO

The M40 is a bolt-action sniper rifle used by the United States Marine Corps. It has had three variants — the M40, the M40A1 and the M40A3. The M40 was introduced in 1966. The changeover to the A1 model was completed in the 1970s, and the A3 in the 2000s. The A3 model costs $3,200.

RPG-7
Capacity: 1, 40mm Rocket

The RPG-7 is a widely-produced, portable, shoulder-launched, anti-tank rocket propelled grenade weapon. Originally the RPG-7 and its predecessor, the RPG-2, were designed by the Soviet Union.

Baseball Bat
Capacity: 1

A baseball bat is a smooth wooden or metal club used in the game of baseball to hit the ball after the ball is thrown by the pitcher. It is no more than 2.75 inches in diameter at the thickest part and no more than 42 inches (1067 mm) in length. It typically weighs no more than 36 ounces (1 kg).

Desert Eagle - Pistol
Capacity: 7, .50 Action Express

The Desert Eagle is a large caliber gas-operated semi-automatic pistol manufactured primarily in Israel by IMI (Israel Military Industries) for Magnum Research, Inc. Final refinements and a production ready pistol was manufactured by Israel Military Industries (IMI), under contract to Magnum Research, Inc.

Glock 17 - Pistol
Capacity: 17, 9x19mm Parabellum

The Glock 17 is the name for a family of semi-automatic pistols designed and produced by the Austrian company Glock GmbH from Deutsch-Wagram, founded in 1963 by engineer Gaston Glock to manufacture high-strength synthetic and steel components.