I'm travelling from the USA to Madeira Portugal and I have a layover for 16 hours. I was wondering if I need a Visa in order to stay the night in a hotel at the airport in terminal 4? This is the first time I'm travelling international so would like some advice also am I allowed to do a little sight seeing while I'm there without a visa?
The situation you have described, and assuming you are travelling on a US passport is covered by paragraph 23A of the Immigration Rules, which says in part...
23A. A person who is not a visa national and who is seeking leave to enter on arrival in the United Kingdom for a period not exceeding 6 months for a purpose for which prior entry clearance is not required under these Rules may be granted such leave, for a period not exceeding 6 months.
Determining whether or not you are a so-called 'visa national' is a matter of looking for your country in the list.
- Subject to paragraph 2 below, the following persons need a visa for the United Kingdom: (a) Nationals or citizens of the following countries or territorial entities:
(followed by a long list of countries)
The US is not on the list and so US citizens are ok to enter on the basis of a successful landing interview.
Finally, as a US citizen, you'll have a landing interview which will cover Paragraph 41 of the Immigration Rules. This says in part...
- The requirements to be met by a person seeking leave to enter the United Kingdom as a general visitor are that he:
The 'formal' and 'official' visa that you get is called 'Limited Leave to Enter', and is issued according to Paragraph 42 of the rules (same link). It is a visa and subject to all the various rules about overstaying, working, and so on.
All links in this answer lead to the UK Government's site so they can be taken as authoritative. Note that SOME of these rules, especially Paragraph 41, will change between now and April 2015.
Not if you're traveling with a US passport. Americans can get their visa issued upon arrival in the UK.