First, Silvest comes with base 13000 power and a continuous skill that as stated, prevents him from being in a deck after September 21st. And second, Silvest gains +10000 power when attack a vanguard if the number of Great Nature rearguards is three or more. This latter skill only works in the vanguard circle, and already it brings to mind several skills of the past.
Grade 4 bears many similarities to the ever-propagating crossride family of skills. Like crossride units, Silvest is intended to have 13000 power whenever possible. His autoskill is a direct modification of Blaster Overlord's, dropping the +1 critical gain and its included cost altogether in favor of simply meeting a requirement. While Silvest has no specific support cards in line with the Flame of Promise or Apocalypse Bat, his skill compensates for this by giving him the equivalent of a 10000-power support, allowing for 23000-31000 power attacks in the vein of Great Daiyusha or Blaster Overlordt. All these point to crossride--a type of unit directly designed to be ridden over a grade 3--being a prototype of grade 4 mechanics, with Silvest being Bushiroad testing the waters for how it would be implemented.
However, crossride in general is the better mechanic. There are inherent flaws with Silvest which in general make him overhyped and underpowered.
First and foremost, grade 4 would harm grade ratios. Riding consistently from grades 1 to 3 across consecutive turns is an important element in cardfight. Not reaching grade 3 at the same speed as the opponent means losing card advantage and subsequently, control of the fight. Trying to incorporate grade 4 into the deck is nearly impossible to balance because a fourth grade was not designed as part of the system. The most optimistic grade ratios are 17/13/10/6/4 or 16/13/10/7/4 with both making room for missing grade 1 very easily, and the latter assumes the use of a cycling trigger unit that hurts advantage in the first place. Implementing grade 4 in the way that past grades are currently implemented would mean raising the deck size from 50 to 60, and mandating a new universal deck size would deeply wound past Tsukuyomi and Beast Deity builds, while giving artificial power to draw- and vanguard-stand based decks that once had to be careful not to deck out altogether. Changing the deck size would be introducing a fundamental rule change to Cardfight!! for the first time, such that we could not longer call it Vanguard.
Second, riding grade 4 is in the first place a bad idea. While Silvest is powerful in Great Nature, similar units would be disastrous to the other clans' existing strategies, and he's already difficult to use in a Great Nature deck. The primary reason for this is that outside of superior ride, you can only ride units of equal or higher grade than that of your vanguard; riding Silvest locks the player out of Leo-pald, Bison and the Guardian of Truth, all units which are intended to exist in the vanguard circle exclusively. Furthermore, Leo-pald and Guardian of Truth generate card advantage with Hammsuke and other rearguards at such a rate that each of them is preferable to the Epitome of Knowledge. Silvest's real use is as a nearly-invincible 13000-power rearguard, but he can't be called without riding him. This makes the Epitome of Knowledge into something the class clown might use, even moreso from a clan that has room for a crossride of its own for Lox. Grade 4 is not an impossible mechanic to implement, but seeing as its prototype already does the same job more effectively in the current climate, until this mechanic has a better proof of concept, it's probably best to leave it alone.
As some closing notes on Silvest, his skills could be further rebalanced by adding a -2000 clause if the player has any non-Great Nature rearguards, and an additional -2000 for being in the rearguard circle.
