In spite of recent communication from the Government, I have not seen any statutory instruments pass Parliament referencing Tier 4.
The statutory instrument (SI) in question is the Health Protection (Coronavirus, Restrictions) (All Tiers and Obligations of Undertakings) (England) (Amendment) Regulations 2020 (S.I. 2020/1611).
Much of this is framed as an amendment to the Health
Protection (Coronavirus, Restrictions) (All Tiers) (England) Regulations 2020 (S.I. 2020/1374).
Like almost all of the SIs used during this pandemic to place restrictions on the public, it invokes section 45R of the Public Health (Control of Disease) Act 1984, which states that:
The instrument may be made without a draft having been laid and approved [...] if the instrument contains a declaration that the person making it is of the opinion that, by reason of urgency, it is necessary to make the order without a draft being so laid and approved.
An SI made in this way must be approved by both houses of Parliament within 28 days (excluding any time both houses aren't sitting for more than 4 days).
Some speculation implies a vote will take place in January, but as of this time Parliament is in recess.
Indeed. Because section 45R was used for this SI, both houses must vote on it in January, otherwise it will lapse after that 28 days. Note also that, under subsection (5), if either house votes to reject it, the SI still has effect up to the end of the day on which it is rejected.
Is Tier 4 just guidance, and if so, could it retroactively be made Law?
It is law, as specified in the above SI. As for retroactivity: although in theory Parliament could do that (and very occasionally has), ex post facto laws are generally regarded as a bad idea.
A complete list of UK Coronavirus legislation can be found on a dedicated page at legislation.gov.uk, and (for SIs specifically) the Hansard Society's Coronavirus Statutory Instruments Dashboard.