Hi PongFu
No Don't have a Tetra - but I had a quick look at the manual - check the nrpn info and tables at the end . . .
We can using running status which means we don't have to retransmit some redundant info.
So you just need to send the info it details - convert the binary to hex or decimal as you wish . ..
It helps to use a calculator that can do the decimal, hex, binary conversion for you
Check you are setup to send and recieve nrpns in global parameters
Then go through the Program Parameter table . .
LFO 4 Freq is param 57 which is for layer A is nrpn 52 and takes 0 -166 as values same as LFO 1
0-166 = 0 - 150 unsynced frequencies then 151 to 166 are the synced divisions . . .
Have a look at what values are being sent when you twist the knob - you need to mimic that range . .
So it's probably better to use midiout and send the data described
ie something like - midiout(0{0xB0,99,0,98,52,6,val_msb,38,val_lsb})
So it just comes down to how you want your interface to work and how to map the values properly
Once you have selected an nrpn you can use the value increment/decrement messages as well
Also remember to use the reset message - looks like its 37 & 36 with 127 as the data
Hope that makes sense
Cheers
MM

