Usually, crit-build [Tetra] is a terrible idea. However, when I rolled this riven, I decided I'd give it a go, and hey, it works pretty alright for an MR 4 weapon. Would I recommend more than 3 forma if you don't have a riven? No, I wouldn't. Would I recommend any forma if you don't really care for the feel of the weapon? No, of course not, this weapon is not very good, so if you don't enjoy it, don't bother putting investment into it; the only reason to really use this is if you like it. If you are looking for a really good laser machine gun, go for the [Supra] (MR 10, from Energy Lab) or, better yet, [Supra Vandal] (MR 14, Baro Ki'teer exlusive). However, if you're like me and really enjoy the feel and sound of this weapon, go for it.
If you do not have a riven for this thing, you have two options: 1) find a riven (which you can do for cheap on this weapon), or 2) use it without a riven. If you do choose to go the riven route, make sure that you get one of two types of rolls: either a roll that would work well with a status build (multishot, status chance, elemental damage, base damage, etc.), or a full-on crit riven, like what I got (crit chance, crit damage, base damage, etc.). Since the riven disposition (which affects the stat values on the riven, and changes based on the weapon's popularity) on the [Prisma Tetra] is almost as high as it can be, even a riven with no negative is viable. However, if you do manage to get a riven like mine with a good negative (- zoom, - impact, etc), then dang, you're lucky, and you should have a much more capable riven.
If you choose to run without a riven, I'd recommend the following build: [Serration], [Split Chamber], [Shred] (you could run [Primed Shred], but I prefer the rhythm of standard [Shred]), [Malignant Force], [High Voltage] (you could run viral, if you use Rime Rounds, but I prefer using corrosive on this weapon), Thermite Rounds, Vigilante Armaments, and Heavy Caliber / Piercing Caliber / Hellfire / Wildfire / Primed Bane mod, with either [Terminal Velocity] or [Kinetic Ricochet] in the exilus (if you decide to use [Heavy Caliber], I would use the latter). Remember, though, without a riven, it's not a very good weapon at all. I would only recommend using it without a riven if you just really, really love it and want to mess with it in lower level content.
Hope this helps someone! :D