What is the maximum length of stay for UK citizens visiting Thailand on a holiday?

I thought you could only stay 180 days maximum

Or the Multiple Entry Tourist Visa, 6 months valid and on each entry you get 60 days of stay.

For 4 months you have two options:

Single Entry Tourist Visa, gives you 60 days on entry, then apply for a 30 day Extension of Stay, after these 90 days make a border run for a new 30 days. OR

visit without a visa and get 30 days, apply for the 30 day Extension of Stay, make a border run for a new 30 day stay and apply again for a 30 day Extension of Stay.

Still you need to leave Thailand after 60days or 90days.