There's really no clean formula for converting between tirz and reta doses. Based purely on weight-loss effect, something like 15mg tirz roughly lines up with 10mg reta. But reta brings side effects tirz mostly spares you from, mainly sympathetic nervous system stuff, elevated pulse, trouble sleeping, plus odd skin sensations like allodynia. Tirz leaves glucagon receptors alone while reta engages them, largely explaining that gap, and tirz hits GIP harder too, which blunts GLP-1's nausea and vomiting, so reta brings more of that as well.
Jumping straight to 10mg is genuinely risky, could leave you fighting rough side effects for a week or longer, and plenty of people here have reported trouble switching even at a modest 6mg mark.
Probably wiser to cap your first reta dose at 4mg and see how it goes, though ramping faster than the usual 4-week gap isn't inherently dangerous, just remember mild side effects tend to worsen as doses climb.