A lot depends on what tech you have.
It used to be that 2 twin turrets massed the same a a single quad, but that is no longer the case. When the roundup for calculating turret gears was removed an 'efficiency' was added to multi mounts (5% for twin, 7.5% for triple and 10% for quad).
It used to be that quad 10cm laser turrets were 20hs but now they are 16.32hs with twins at 8.28hs. Those numbers assume that the turret tracking speed is 4x the current turret tracking speed tech. Additionally that assumes that the fire control tracking tech and turret speed tech are at the same research level and the turrets are intended to be optimized to used with 4x speed fire controls.
Which beam fire control range multiplier to use is also influenced by the tech researched. Fire control range dictates the tohit chance percentages. At level 1 50% is just 10,000km. By selecting 2x the range 10k/km changes dramatically improves to 75% with 3x netting only another 8% to 83% and 4x being 88%. A cheap tech jump to level 2 has 50% 16,000km giving 69% at 10k/km with 2x/84%, 3x/90%, and 4x/92%. Level 3 has 50% at 24,000km giving 79% at 10k/km with 2x/90%, 3x/93%, and 4x/95%. Level 3 is only 6,000rp investment above starting TN level 1. The best "bang for the buck" is 2x range BFC.
I've discussed the tohit chances at 10k/km since the original post was concerning missile point defense. Unless you have a beam fire control and beam that can reach missiles at greater ranges than 5 seconds missile flight set the PD mode to final defense with a range of 1(10,000km) and keep your formations tight for mutual support.