Hi and welcome. :-)

I can't say for sure what you're describing. First of all, your car has 4 gears only if it's an automatic. You have 1,2,3 and 4. What the lockup torque converter does is at around 48MPH, it engages (locks up) so that there is no wasted power in the torque converter due to the power having to be transmitted through the fluid. It's PERCEIVED as a slight upshift, but it isn't really. You're still in 4th. To get a better explanation of this with diagrams and what not, go here.

Now, when you say your engine's RPMs go up, how many are you talking about exactly? Because what I think you're describing is your Overdrive going out, which would cause your car to drop back into 3rd gear. This is the same as pressing the little button on the left side of your shifter knob. If that is the case, it could just be your overdrive relay which is going. It could also be the solenoid, or the switch, but I'd check the relay first.

Instructions to repair/replace the overdrive relay are located here

If you indeed are actually sure it's your lockup torque converter, than forgive me for having assumed you were confused. :-)

Best of luck!
--
Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'