I don't think the Star League is built on a base unit of "27 lances". They're built on a base unit of "BattleMech Regiment" -- based on the fact that the individual BM regiment was the smallest sized unit we routinely see deployed on its own.
This is absolutely the wrong model for a micro-state, and very few of them really work with it. Looking at the Periphery states, we see that they're mid-sized between the Inner Sphere and the micro-states. And many of the Periphery nations work off a base unit of BattleMech battalions rather than regiments (in some ways, the 3025 Capellan Confederation did, as well, highlighting its runt-of-the-litter status). The Concordat, in particular, talks a lot about battalion deployments in place of regimental ones.
Think of it this way; if you're a five-world microstate with four battalions of 'mechs, you're probably not going to group them all together in a reinforced regiment, nor are you likely to have one three-battalion regiment deploying everywhere together, with the remaining fourth battalion doing its own thing. You're a lot more likely to use the battalion as your base unit, so you can cover four areas in relative strength (able to fend off anything but a full attack).
The four-unit Lance makes a lot of sense for the same reason essentially every air force in the world uses a four-plane flight: it can be broken down into two mutually-supporting pairs. While historically tanks have more frequently been fielded in platoons of three or five, and lighter vehicles such as scout/cavalry cars & APCs into platoons of 6+, these factors tend to deal more with the rank range of crew members or the troop-carrying capacity of the vehicles; the former is an administrative issue, the latter an ease-of-operation consideration. Which is a good, military-speak way of saying it all really boils down to personal preference on the part of the organization using them.
Is the four-unit lance perfect? No, but it's a lot better than the ridiculous 2-unit aerospace lance (or the entire idea of attaching aerospace assets directly to a line company). The four-unit lance makes for quick 4-on-4 BattleTech games, despite the fact that the idea of sending four units alone and unafraid to hopefully run into four units of the enemy violates 3500 years (by the 31st century) of military theory and practice. For my money, I think the battalion should be base (that is, individually deployable) unit, but only because the number of regiments in BT canon is ridiculously low for the population size & industrial strength of the states involved.
The four-unit lance gets the job done, is easy to combine with other lances, is reasonably capable of self-support and of achieving real combined-arms dilemmas, but should never be deployed without the other lances of its company being in range to provide support.
