You will likely have to sign some form of contract to sign up again for 4G service.
They’ll give you a new 4G SIM card (keep your 3G SIM card in case your contacts are on that card, for which they can help transfer contacts from SIM to SIM).
My CHT bill went from NT$183 per month in the most basic/cheapest rate for 3G to now NT$199 per month for 4G (500MB of download per month! Yeah, so much, ha ha. 20mins of calls or something).
I had choice of signing 1 or 2 years.
I believe 亞太電信, Asia-Pacific Online, has a new deal just this week (八八節).
NT$377 for 7G of download per month.
Otherwise, go with any of the other telecoms for like NT$499 per month of unlimited (signed contract, likely 2 years).