I think if you're able to get that leak stopped (& top off fluid as needed), you'll get 4th again.
Besides the 1-2 shift, the servo also has a function in the 3-4, as well. However, it depends on sufficient fluid pressure to work correctly.
Make sure the O-rings are seated correctly. Also, check the free play with the servo - it should not have more than about 1/8" travel. If you've got twice that or more, then fluid can get past there (and you'll have lower fluid pressure, etc.) Usually, the excess play comes via wear - but since this is a new servo you put in, that shouldn't be an issue.
The fact that this happened right after you swapped the servo points to that as the cause, IMO. A couple of things you can test...
With the vehicle 'overnight cold' (8hr or more), check to see if you get 4th the first time you go through the gears after starting it up. Fluid pressure is higher, when the fluid is cold. You'll likely need to top up before starting it, especially if you've lost a significant amount of fluid during your testing. If you get 4th then, but not as the fluid temp rises, then you know it's a pressure issue.
Try letting up on the gas around the point where you think it would normally shift to 4th. Sometimes the trans will go to 4th, but not be able to hold it (again, a sign of either low pressure). Or, you could have leakage in the 4th accumulator (but you inspected that for wear when you had the VB and separator plate down.) Since the 4th accumulator pin sleeve is a blind hole, that leaves only the tolerance between the sleeve and the pin itself.
Finally, you can try swapping your old servo back in (you'll need a new o-ring or two, as the old ones are worn and / or damaged - and the ones from the new servo may be too large to fit properly on the old one), and see if that stops the issue. If it does, then you'll need to find out the difference between the two of them (assembled correctly? piston out of round? etc.)
Again, my disclaimer - I'm not a professional mechanic - just an owner with a little experience (which can be dangerous in itself.) But I hope you find the cause of the leak and are able to stop it.