You don't even need to justify it by saying that it's a metabolite -- it's well know that putting a hydrophobic atom into the 4-position increases an amphetamine's affinity for SERT and thus serotonin release, which is well known to be a precursor for amphetaminergic neurotoxicity and cardiotoxicity.
EC50 DA/NE/5HT nM
d-Amphetamine 8.0±0.43, 7.2 ±0.44, 1756±94
meta-Methylamphetamine 33.3±1.3, 18.3±1.4, 218±22
para-Methylamphetamine 44.1±2.6, 22.2±1.3, 53.4±4.1
Wee S, Anderson KG, Baumann MH, Rothman RB, Blough BE, Woolverton WL (2005). "Relationship between the serotonergic activity and reinforcing effects of a series of amphetamine analogs.". J Pharmacol Exp Ther 313 (2): 848-54.
The affinity of methcathinone and methamphetamine for DA/NA/5HT release is as follows:
EC50 DA/NE/5HT nM
(+)-Methamphetamine 12.3 ± 0.7, 24.5 ± 2.1, 736 ± 45
(–)-Methcathinone 13.1 ± 0.6, 14.8 ± 0.4, 1772 ± 160
Rothman RB, Vu N, Partilla JS, Roth BL, Hufeisen SJ, Compton-Toth BA, Birkes J, Young R, Glennon RA. (2003) In vitro characterization of ephedrine-related stereoisomers at biogenic amine transporters and the receptorome reveals selective actions as norepinephrine transporter substrates. J Pharmacol Exp Ther. 307(1), 138-145.
We can only make an educated guess that 4-Methylmethcathinone is somewhere between methcathinone and 4-Methylamphetamine in terms of 5HT release, and given that it's very possible we're dealing with a strongly neurotoxic and cardiotoxic agent. I mean, methamphetamine is incredibly neurotoxic yet is 60x preferential for DA over 5HT compared to 220x preferential for amphetamine, while 4-Methylamphetamine's ratio is 1.2x!