from the pictures, i am pretty sure its a .21, 9 cooling head fins, iirc the 4.6 had 10.
the only real difference besides the engine itself was the 1-2 speed pinion gear is plastic on the .21, and steel on the 4.6.
(have not had one strip yet, through .21, .25, and .28 engines. if it does just replace with the 4.6 variant one though.)
might be worth a read if you get it:
http://csgbenefits.org/mgt/ - have semi recently actually emailed the guy that keeps the site going, was a really cool guy and was even talking about maybe coming back to the hobby.
if you plan to keep the stock radio setup, move the battery plug in to channel 4 instead of battery. (battery is BECd to 5v.) and get a 5 cell RX hump pack (recommended regardless).
my first biggest recommended upgrade is a FOC (associated part number 25415 / ASC25415), and get RC raven dual Rate springs (ones for the t-maxx fit as well, and a bit tighter.), or get a set of 4 red AE MGT springs and add one per corner.
if you are going to go further with mods on it, a side header, and tuned pipe really wake these engines up, i was told the Losi LST stock pipe are one of the best cheap options to unlock more power out of em.