Bitcoin Core is such a cool software.
It indeed is. Also note, that testnet4 is still in its infancy. So, you can get your transaction confirmed even for free, as long as there is enough space in blocks. You can check your peers, maybe some of them accept free transactions?
$ ./bitcoin-cli -testnet4 getpeerinfo | grep minfeefilter
"minfeefilter": 0.00000000,
"minfeefilter": 0.00000000,
"minfeefilter": 0.00001000,
"minfeefilter": 0.00001000,
"minfeefilter": 0.00001000,
"minfeefilter": 0.00001000,
"minfeefilter": 0.00001000,
"minfeefilter": 0.00001000,
"minfeefilter": 0.00001000,
"minfeefilter": 0.00001000,
So, if you spot a node with "minfeefilter" set to zero (or anything lower than 0.00001000), then you can try to send a cheaper (or free) transaction, and there is a chance, that you are connected to some CPU miner, who will confirm it for free.
For example, for testnet4, I have these settings:
mintxfee=0.00000001
minrelaytxfee=0.00000000
blockmintxfee=0.00000000
dustrelayfee=0.00000000
mempoolexpiry=35000
acceptnonstdtxn=1
maxtxfee=1000000.00000000
mempoolfullrbf=1