Faulty torque lock up in 4th gear

neither
the torque converter clutch solenoid / servo will not give any drive action if faulty
the transmission if operating fine in all gears with no noises will not need replacing
save money by going to an accredited transmission specialist shop and have the fault codes read first then replace the problem sensor , servo or solenoid

sometime the gear inter mesh is not accurate.

P0734=A/T 4th gear incorrect ratio. This has nothing to do with the speed sensor. If the speed sensor was faulty, it would also set OTHER gear incorrect ratio codes. My experience tells me the 4th gear incorrect ration indicates the torque converter clutch (TCC) is slipping and nothing more. Unless there was a speed sensor DTC (Diagnostic Trouble Code), the sensor is not the issue (yet). The fact P0734 was set (trust the computer), the TCC is slipping (out of range). Question: Maintenance history may indicate the wrong transmission fluid may have been introduced. Hyundai's Transmission fluid is strictly SPIII... there is no substitute. The incorrect fluid will cause 4th gear slippage. Now, having said that, it doesn't mean the 4th gear (lock-up) clutch isn't worn out. Have a qualified/certified Hyundai Service Technician properly diagnose this issue.

Transaxle is worn out and slipping

Torque Converter Clutch may also
be slipping in 4th & lock-up

the rachet and lock on the handbrake handle itself is faulty and either needs 'loooking' at or replacing. leave the car in gear in addition to the handbrake to make sure ur car doesnt do a runner while u are waiting for repairs.

No, this is a situation that involves one or more of the shift solenoids that are inside the pan. These shift solenoids are malfunctioning at this time and, i recommend inspecting the transmission asap for faulty solenoids.

DTC P0734 is "4th gear incorrect ratio" - that's your torque converter 'LOCK UP Clutch" It's worn out (or you're using the wrong Trans Fluid required by Hyundai - SPIII OR Both). The P1624 is simply the Trans Control Unit (computer) communicating to the Engine Control Module (ECM computer) MIL - on request signal to ECM = asking it to turn on "Check Engine" light. MIL = Malfunction Indicator Lamp. When the torque converter 'lock up" clutch slips or doesn't engage, the trans automatically returns to 3rd gear and stays there (it's called 'fail safe' mode) until you cycle the ignition key, then everything resets until the malfunction occurs again.
You'll need to have an AT (Automatic Transaxel) tester hooked up to verify this. Hyundai has a Service Bulletin specifically for "gear incorrect ratio' codes. If when you changed the trans fluid, was it done at a Hyundai Service Center? If not, someone may have installed the wrong fluid.

you may be feeling the torque convertor locking and unlocking....if you feel this on grade it may be normal. there is also the possibility that the brake switch may be acting up and causing it

Use it only on highways and you should wait until you are in 4th gear to turn it on. If used in a moutain area, it will put more strain on the transmission so make sure it is not on while going up hill.