About 6 weeks ago, I noticed my clutch was not working properly, as it would not return once I took my foot off or I just could not shift into any gear. Had a repair shop tighten the slave(?), and also refilled the brake fluid. But after this, when I would shift into 4th, it made a horrific "grinding" sound at times. It would also pop out of 4th gear when I would shift. Now, a few days ago, I no longer have 4th gear.
12st, 2nd, 3rd, and 5th work fine, although 3rd is starting to behave like 4th gear.

Am I losing my clutch?

Thanks in advance.

No adjustment on the clutch linkage - or slave cylinder by the way. You might have a bad master cylinder if the clutch doesn't return right. Have you been losing a significant amount of brake fluid? Is the insulation under the carpet by the clutch pedal wet at all? Any noises from the clutch pedal? If you are answering yes to any of these questions, think hydraulic components before the clutch itself. A lot cheaper than pulling the engine.

How many miles are on the car?

AJU:

The clutch returning has not been a problem. When it has stuck to the floor, it was low on brake fluid. The fluid loss has not been as noticable as before. The carpet does not appear to be wet. The leak comes from the engine compartment (driver side on the right of the engine, he said it was the slave?) The clutch pedal does not appear to make any noise.

So the slave cylinder was replaced when the pedal didn't return? Are you sure that all of the air is out of the system? If there is even a small bubble left in the line, the clutch may not release completely, which can cause grinding of synchros. Also, if you changed your slave cylinder, change you master cylinder as well. If one is old enough to leak, the seals in the other are not far behind.

Nothing was replaced, just tightened (I am sure he bled the air). Would this cause a problem if I was low on fluid and topped it off?

probably..the clutches are auto-adjust.
Mine started to feel 'strange' for about a week then crunched all the gears.
Replacement is an engine-out job, at least 8hrs labour, and make sure you get the rear oil seal replaced at the same time given they've almost got down to that part already.