It seems you are confused between futures and cex/spot if you are going to futures there's no list of pair for USDC to USDT because these two coins are stable coins. Even in other exchanges, you can't find this pair because both of them are stable coins. Unless you trade on spot/cex to place a limit order, you can also directly convert USDT to USDC and vice versa.
Stable coins in theory need to be stable, and even they have risk of depegs. There is very little demand on futures trading on volatility of stable coins like depegs. It makes senses if centralized exchanges don't provide trading pairs between stable coins in Futures Trading market.
Conversion takes fees and with only $4, it doesn't worth to make any conversion. Some exchanges have their min trading like $10 or at least $5, so with only $4 the user will have to forget about trading.