I was playing an online X-Wing game. It let me do this. I have a target lock on Chopper and I have a focus. I shot Chopper, then used my Focus, then it said I had 2 options, I could use my Target Lock OR use the Target Lock from R4. I used the R4 TL, and it kept my other one. Is this legal? I thought you could only have 1 TL at a time.

Yeah, that was not correct. Unless you have Weapons Engineer or are Redline, you can't maintain more than one Target Lock (and in the case of Weapons Engineer, you can't have two TLs on the same ship, so you still couldn't get to where the online game put you).

Umm... not really, kinda.

If you have R4 Agromech equipped, and you have a TL and a focus when you make the attack, you could do the following:

Reroll, then spend focus, then use R4 to gain a TL.

Reroll, not spend focus, end up with no TL.

Spend focus before rerolling, gain a new TL but discard the old one, after which you could reroll using the TL you have now.

R4 Agromech doesn't give you an extra TL, it replaces the one you have if you have one when you use it. Benchmark (which I assume is the programme concerned since Vassal and TTS don't automate this stuff) is not a rules source and often gets stuff wrong.

As @thespaceinvader said you can use R4 Agro as a clunky FCS if you spend tokens in the right order as in the first example