While I agree it is not good practice to run things beyond their rated potential, why do you percieve nearly 100% chance of failure? These things must be slightly overdesigned, no?
How about 4500w?
cheap products from china are never over designed. more often they are
under designed, and often, a product that should really only be rated at 3500w they slap a 3800w sticker on. dont expect miracles out of bare-bottom cost chineese made products.
that being said (and ignoring possible inflated ratings from the manufacturer because theres no way to easily know), you always need to assume 3800w max means exactly that. 3800w does not mean 4500w, nor 5000w. if it did, then they would rate it higher because higher specs sell better. do not ever expect to be able to go over somethings rated max without issue. doing so is entirely at your own risk (risk of money loss, personal injury, or property damage).
when it comes to semiconductors, like these, or SSRs or any type of mosfet, the closer you get to the rated maximum, the
exponentially more heat they produce (its not linear). so if you have a 1000w load to switch, buying a 1000w max rated SSR will run very hot. buying a 2000w max rated SSR will run much cooler, and will last that much longer. if you put a 1500w load on a 1000w max SSR, its going to burn up very shortly, if not instantly.
The 3800w dimmer would work if you use a SSR also, right?
thats like saying "this light switch would work if you use a light switch also, right?"
what do you mean?
So rather than building some elaborate control box with PIDs and SSRs, I could use this "dimmer" on the hot lines (it appears to have 2 ins and 2 outs) of a 4500W/240VAC heating element and have the ability to dial down the heat once I've reached a rolling boil?
if you want to do this you would only use one dimmer rated for the full load of the element, and it would be on either of the hot legs; not one on each side, if thats what you meant. but yes, that would work.