Is Paul contradicting himself about the observance of special days in Romans 14:5-6 and Galatians 4:8-11?
In short, no. Paul is speaking to two different peoples, as the names of the books suggest the Romans and the Galatians.
In Romans, Paul opens the chapter speaking to the Romans about those new into the Christian faith. Newly converted ones would still have in mind their old ways of worship, either the Mosiac Law or pagan worship. Paul kindly reminds them to accept these new ones:
"Welcome all the Lord's followers, even those whose faith is weak. Don't criticize them for having beliefs that are different from yours."–Rom. 14:1 (CEV)
This can also be seen in the issue of circumcision in Acts 15:1-5. Verse 5 even indicates:
Then some of the believers who belonged to the party of the Pharisees stood up and said, "The Gentiles must be circumcised and required to keep the law of Moses."–Act 15:5 (NIV)
Some Pharisees that had come to accept Christ were still trying to uphold the Mosaic Law. So, in essence, Paul in Romans 14 is saying not to be harsh with someone that is newly converted and realize that it will take time for them to adjust to the Law of the Christ.
As for the Galatians, Paul is talking to Christians that had accepted and were Christians for some time.
"Formerly, when you did not know God, you were slaves to those who by nature are not gods. But now that you know God–or rather are known by God–how is it that you are turning back to those weak and miserable forces? Do you wish to be enslaved by them all over again?"–Gal. 4:8, 9
The Galatians were going back to their old ways after having known God and as Paul puts it "known by God". They were reverting back to nonsensical things as Peter referred to:
Of them the proverbs are true: "A dog returns to its vomit," and, "A sow that is washed returns to her wallowing in the mud."–2 Peter 2:22; Prov. 26:11 (NIV)
It helps us to see that the consciences of others need to be taken into consideration before we speak harshly. And that we need to stay firm in what we have learned and not fall back into our old ways.