Oddly, some effects will not be very noticeable if the planets are stipulated to be in the same orbits. The gaps in the asteroid belts are an effect of orbital resonance rather than gravitational pull, and the Jovian "Trojan" asteroids are going to be in the same spots because the L-4 and L-5 points are determined by the relative positions of Jupiter and the Sun.
On the other hand, massive giant planets will have a much greater retinue of moons, rings and orbiting debris because of the greater gravitational pull. Jupiter will probably be a "Brown Dwarf" at 4.5X its current mass, so will also be a very strong source of infrared radiation, and moons like Europa might not have an ice covering (in fact, the water covering may have evaporated away since there would be no ice cover and the gravitation of the moon would be too small to prevent the water from evaporating away). Saturn may also become a brown dwarf, with similar effects on Titan.
Spacecraft will be in grave danger trying to carry out gravitational slingshots since they will be entering a system with thousands or millions of small bodies in orbit, making the risk of collision quite high.
Ancient peoples will also have been able to see the giant planets with the naked eye from antiquity, so there would have been 8 visible planets, with the changes to mythology and astrology that that would cause.