I take my amateur photos with a manual Yongnuo 560 IV, and I am quite happy, but a lot of times a second speedlite could be very useful. I am considering to buy a TTL Flash to try it out. My only concern is that I am not sure how my existing flash would work with a TTL one.
Is there a TTL flash and trigger combination that would work with my current flash?
I guess the current one could not work in slave mode because of the pre-flash.
Depends on your definition of "work".
In the Yongnuo system, their three separate triggering systems (560/60x, 622, and RT) are mostly incompatible. While you can purchase a TTL flash/triggers and use them in 603 mode with your YN-560-TX, using 603 mode pretty much renders your TTL/HSS gear as manual-only gear.
If you use the add-on 622 TTL/HSS triggers, you'll have full TTL/HSS/power control over the new TTL speedlight, but you lose power control over the YN-560IV (which can only be power-controlled from a YN-560-TX). Unless you switch the triggers to 603 mode, which then is identical to using YN-560IVs with your YN-560-TX. Sync and power control, no HSS/TTL.
If that's okay to you and you only want TTL on the hotshoe, then most of the Yongnuo flashes with names that don't end in 0 do TTL, and anything that's 500 series or later is likely to have 603 mode, although you will want to check the individual model specs. Flash Havoc is a great website for that. (See also: What are the Yongnuo flash naming conventions?).
However, if you want to mix TTL/manual only gear with full function on all gear, and without going all-TTL gear, you need to swap out of the Yongnuo system altogether: it just doesn't work that way. Luckily, others do. The Phottix, Godox, and Cactus V6II systems allow for mixing of manual and TTL gear in the same triggering system. Godox, however, is the only system with similar pricetags to Yongnuo gear. And, of course, cheap Chinese flash technology changes very rapidly, so if you're reading this answer post-late-2017, do more research. Every brand I mentioned may have been superseded by something else.
I guess the current one could not work in slave mode because of the pre-flash.
You can use the YN-560IV in S1 or S2 mode with another flash using radio triggers, but not with flashes using a "smart" optical system. "Smart" optical systems, like Nikon's CLS system or Canon's optical wireless TTL system, use multiple pre-flashes. But if you use radio triggers, that system isn't being used.
The S1 mode will fire the 560 on the first flash burst sensed (if M mode is used on the radio slave); and S2 mode will fire the 560 on the second burst sensed (skipping the metering pre-flash if the radio slave is in TTL).
