Generally speaking, it is not the valve cover gaskets that leak. It is usually the breather covers (which sit on the valve covers) that leak; the sealant used is RTV.

If the actual valve cover gaskets are leaking, try retorquing the bolts that hold the cover on. USE A TORQUE WRENCH so you know how tight they are, those bolts will break off if you go too far with them.

If it is the breather covers, then you must remove the breathers from the valve covers, clean off the old RTV from both surfaces, reapply RTV and smooth it out so there are no bubbles (and use ONLY enough to fill the valley, "more" is TOO MUCH). Then you put the breathers back on and tighten the bolts.

If you got four years, you did good. I did my own (you will see my posts in that thread) back in 2015 or maybe 2016. Just this year I had some oil seeping from around the driver's side breather - I made it over eight years. But that's because it is hard to do a good job by just removing the breathers and reapplying RTV, I went way farther and did it RIGHT:
1) I did the work myself.
2) I used Max's "tongue depressor method" of applying the RTV.
3) I put probably 6 hours in the project, including:

• taking all covers off the car, separating them, cleaning them with gasoline (to get all the RTV off)
• Then cleaning them with Purple Power to completely clean all oil/gasoline off
• Then I left all four parts separated and sitting in front of a fan overnight so all the tunnels and crevices are dried out.
• When applying RTV, I applied ONLY enough to fill the valleys with RTV, then scraped excess off with a tongue depressor, then let the RTV sit for maybe five minutes to set up a bit
• I put the breathers on the valve covers and tightened the bolts only to their spec (it is not much), let them sit for a little while
• I then put the breather/valve cover assemblies on the car and used a new set of bolts from NEEDWINGS, along with new valve cover gaskets from NEEDSWINGS.
  DONE!

No garage is going to do what I did, because they could not do so without charging you at least $750 labor. If you insist on paying someone to do it, if they got you four years before, I'd go back to them.

CAN YOU DO IT? Well, if you read that thread, you can tell the most "Mechanic" work I've ever done, was that job right there. It's the most I've taken on in my life insofar as working on a car - and eight plus years later, I am happy I did it. Keep in mind I started on Saturday morning and ended up leaving the thing torn apart till the next weekend - you should be able to move faster that that, but I saw no reason to. I gave the parts overnight to dry out and took my time applying the RTV, etc.

Hey, one thing, my oil cap is leaking, so you might look at finding a new gasket for the oil cap seat and make sure you tighten the bolt under there really well before putting it all back together.