It is probably a good idea to wait with the secrets until all the platforms have been raised. Or you may smell burnt socks.

At the first platform, as soon as you have raised the bridge to the next, a small walkway rises right next to the start of the platform. Follow it to the first secret.

After #1, carry on along the corridor. You reach a window, where you can jump down to the platforms again. Don't. Look left. From the windowsill, you can do a slightly tricky jump over to the next set of shelves in the cliff wall.

When you have secret #2, i suggest you proceed with #3 and #4 at extremely high speed. Haul ass, marine.

Straight down from #2 and a bit to the left, a hole in the cliff wall. Go there. There is a face on the corner of the platform; it calls a lift out of the lava.

Dive into the lava here to get the last secret. You have a considerable way out again, so you better leg it.