What is the best heavy-duty tactical folding knife available?
With a blade around 4"...
Esav is right - without a practical, universally accepted definition of tactical, the question serves no purpose other than to stir up responses.
Being so vague, the OP is really revealing they know little about knives and are just looking to buy an impressive toy.
"Best?" generally includes the top of the line models designed to meet the genre criteria (more later). They have to be well made, of quality materials.
"Heavy Duty?" Big, thick, massive, and capable of taking abuse.
"Tactical?" The
style is compatible with military use. Non-reflective, quiet, and designed for the unique environment of outdoor use in difficult or unusual circumstances.
That can exclude some knives quickly when specific jobs come to mind: cutting live electrical wiring up to 240V, chipping sniper holes into masonry construction, working with chemical, biological or radiological agents, or being decontaminated. After all, since your weapons can be deconned with caustic bleach, your pocket tools should be, too, with no residual absorption or entrapment of contaminants.
This is where "tactical" knives part from "Tacticool." Grip materials that can draw liquids into the matrix through capillary action are not good. That can leave out most micartas, carbon fibers, fiberglass laminates, etc. The fibrous layers act as resevoirs for contaminants.
Conversely, any grip that conducts electricity is no good either. Hence the insulated handles and scabbards on some bayonets.
So, if you can't cut live electrical wires without further protection, or can't guarantee the knife safe from nerve agents, it's not tactical. It can't support modern military use.
That makes most folders on the market marketed as "tactical" pretenders to the throne. Of course, any knife that hints of military use is already suspect unless it has a NSN for issue - and most of those are not specified as combat tools, just MOS accessories, usually for non combat jobs.
Fortunately, tactical knives do offer utility in a non-dressy style that resists looking worn or dated, in a abuse resistant construction, and in more patterns than we've seen get invented since the early 1900's. They obviously fill a need the regular polished blade, natural handled and traditionally contructed knife doesn't meet.
The trick is to understand most tactical knives aren't.