Cerebral Boosters are a good fit for magicians with Logic as their drain stat, like hermetics. Besides the ding to Essence their biggest downsides are their high cost and Availability, but if you can swing them with Transhuman Prototype then so much the better.
A magician doesn't need Magic 6 to be effective. Low force spirits are excellent skirmishers and can distract opponents, and some of their powers (like Movement, Concealment, Innate Spell) can be highly beneficial even at Force 3.
Similarly, there are a ton of spells that are useful even if they're not Force 6. Detection spells, for example, max out their utility around Force 4. Higher Force can get you a better range, but an Extended spell with a range of Magic 4 * Force 4 * 10 meters = 160 meters is still hugely helpful. Lots of Manipulation spells are great at lower Force too, like Levitate, Magic Fingers, and Fashion.
Like Beta says, a Magic 4 magician is going to play differently than a Magic 6 magician, but they're still very useful. Their emphasis might be on utility spells, rather than "I can kill you with my brain", but there's still a lot of value as a support role.