On stage 4 of R-Type there are lots (and I mean lots) of small green circles with red dots in the middle. These are spores, and over the course of the stage there are 5 of these hidden away in places that are hard to reach without dying.
I suggest you play the game on 'infinite mode' in order to make getting these much easier, as most of them are next to a wall on the left hand side meaning getting your ship in front of them to destroy them is almost impossible. On infinite mode you can simply die and fly over the scenery while you are invincible after respawning.
1. At the very beginning of the level you will spot two vertical rows of spores. The hidden spore is the one right at the bottom on the left hand side.
2. After the first gap in the scenery, you will spot a single sport directly followed by a vertical row of 4 spores, the hidden spore is the very bottom one in the row of 4.
3. You will soon come to a section completely filled with spores, in this section there will be alcoves in the scenery at the top and bottom filled with even more spores. The third hidden spore is in the first alcove at the top, the very top of the screen. It is the top spore on the very left hand side. It can be difficult to get, but if you kill yourself and then charge up a shot while still flashing, the charged shot will be big enough to tag it for you.
4. The fourth hidden spore is just below the third, in the bottom alcove, again on the very bottom, the left hand side of the alcove.
5. Just before the boss, just after spore number 4, there will be two rows of 8 spores, the hidden one is at the very bottom on the left hand side.
If you don't manage to find them all in one run, don't worry, the game will still count them. It's even nice enough to have the ones you've already found flash red from then on, so if one of them is flashing red it means you have already found it.