I want to create a lab item that gives +4 to Perdo
Which lvl 40 effect can do this?
I'd like to use In Re or Pe as techniques and Vi Imor Me as forms.
That's a bit tricky. For an item to function - IMHO - as a straight bonus like the one for Perdo, the effect would have to be one that would apply in most of the Pe activities in the lab. Symbolically speaking of course, not physically, since it would otherwise be hard to justify any bonus for teaching or inventing spels.
It's particularly tricky with the incorporeal arts. Had it been an item using Perdo Terram or Herban, it would be one that destroys said things while you invent, giving you a sympathetic bonus.
I'd say a Pe Im device which destroys or lessens the sensations of various objects could work. By repressing one species, the other come through more clearly, giving you an understanding of this.
I've only ever made one such object for a lab, and that was the pretty straight forward Bookstand of Hespera.
A sink or black hole, that destorys anything thrown into it. Or a space that makes the item age at a speed of a year per 10 seconds or so. Wither and die, become dust. Call it "the eye of Balor" or "Belleros". 
RThe first one is handy for cleaning up the lab. And for generating stories when your apprentice throws your lab notes inside due to a teenager angry moment. Great for tytalians 
Cheers,
Xavi
Well, in theory (almost) any bonus can be acquired, but whether a pure +4 bonus is appropriate to an item, or whether it might be mixed with other bonuses, may be up to your SG/Troupe - there is not always a "pure" solution to a narrow problem like this. But let's see...
Under Perdo specialty, the Covenants book suggests: "The lab might be dark or dirty, possibly containing decaying or dead things. It is often an unpleasant place."
The specific Structure/Outfitting examples are all Flaws: Deformed, Vile Surroundings (e.g. a swamp, a hovel), Decaying, Dirty - and Supernatural are a catch-all. Suggested Features include a Cage, Fireplace, Pit, Rack (the torture device), and a "Void". (And some, such as a swamp or the Fireplace, could clearly be used for one or more of several Art Specialties.)
So... an item that mimics one or more of those, or shares characteristics with those, could be a first step. To simply instantly destroy something seems too easy - a mage gains little by watching something vanish. I'd suggest a wide-target Perdo effect with a duration of Concentration, with the Item Maintaining Concentration, that slowly destroys something, so that the mage can examine and observe it as it goes, and thus gain insight into the specifics of the process.
And that's a great image for a Perdo lab. 
And I'd like my old car to run on tapwater, but it just might not be possible.
Perdo is certainly a good start, but I think those Forms are a bit narrow. Not every Form will work for every bonus, even a Technique bonus.
Most Lab Items that I design for my magi almost have to be Greater Items, simply because the mage isn't going to have a Lab Total of 80 to create a Lesser Item with a Level 40 Effect, just isn't going to happen in this decade. (Possibly a Lab Total of 60, and a 70 year item - that is doable!).
But with something this size, a junior mage usually bites the bullet, invests an item with 10-12 vis, resigns themself to 2-4 seasons/effect, and maybe designs two or three useful Lab Bonuses that can be put in the same item. (And possibly then ask around at the next Tribunal, see if you can make use of your lab texts to create a 1-season copy, and sell a copy or three for huge vis returns, if that sounds attractive.)
Here's what I'd suggest, as a solid starting point...
Black Hearth
Pe(Re)Te(An, Co, He) 40 (35)
Very slowly and methodically decays and destroys anything placed within it. This process can be stopped, the item examined, and then continued, subject to the number of uses/day.
(Base 4, +1 Touch, +1 Concentration, +1 Item Maintains Concentration, +1 Rego Req, +3 mags Form Reqs, +5 24 uses/day)
S/M Bonuses: Basalt +3 Perdo, Hearth +5 destruction (plus more if he's Verditius, or can hire one.)
That will decay most any solid material - Animal, Human, Plant, or Earthly solid - up to gems and metals. While that leaves 6 Forms not specifically included, the concept is that such Lab items give indirect but appropriate insight, perhaps to ingredients or resonances that help the creation process in general. (And Aurum, Aquam, Mentem, Imagonem, Ignem and Vim are not the most "concrete" subjects for Perdo'ing. The idea is that it's appropriate, not all-inclusive.)
Altho' "infinite uses" might seem better for a Lab Item, this has an amply long duration (which is the idea), and that's still one use every 25 minutes during a 10 hour work-day, all day long, every day, all season long. For me, that's ample. If a single use takes ~5-60 minutes, and the mage is taking notes and adjusting the effect and doing anything else between uses, that's still probably more than she can use in most days. (And if not, she does something else or takes a break early, and finishes up tomorrow - not a big deal.)
And, as X says, very handy for keeping the lab clean, or disposing of clumsy assistants.
(For a 2nd effect, I'd go with something in the Intellego direction, perhaps, to better observe the decaying process? Might add to Perdo and to another Specialization?)
An item that takes the appearance of an object away from it and then slowly destroys it?
That would make it "cheap" to use and use 3 out of the 5 desired arts...
Hmm, if you do a Wizards sidestep with greater range, and then cast an invisibility spell on it(after placing it in an empty room somewhere hehe), does the magi or the invisible image cast the shadow? 
:mrgreen:
A rusted iron gibbet enchanted with an effect that creates a human corpse. It is sustained for a season, during which the corpse decays. The magus is able to control the manner of the degradation, perhaps one season watching it decay from the outside in, and the next seeing the rot take hold on the inside before making its way out.
The body is modelled on a person of the magus' choosing at the time when the device is designed. Magi with tormenting masters would do well to hide this device should their master even visit.