I miscarried 4 weeks ago. I already have successfully performed a hefsek tahara, and have gone to the mikva. Not long after becoming tahor, I have noticed some spotting.

What are the halachic guidelines for this situation?

The halachic guidelines are the same as spotting in general: As long as stains are found on a colored garment or a disposable liner/pad it’s not an issue as long as it is not a flow. 

The difference between staining and a flow is determined by the amount found, if it reaches around the size of a credit card it is considered to be a flow and would render one nidah.

It is recommended to wear a pad to be able to monitor easily whether the staining has turned into a flow (as mentioned before if it’s just staining it’s not an issue if seen on the pad).

Any stain found on a tissue, a Rov should be consulted as there are many factors and every case is different. As usual it is always advised not to look.

This is just a very basic guideline, when in doubt a Rov should always be consulted.