Robert of Strathearn, who ruled Strathearn 1223–1245, is the fourth known Mormaer of Strathearn, but of course this is simply a source problem and certainly does not mean that he actually was the fourth.
Robert was the fourth son of Mormaer Gille Brigte. Not much is known of his reign, but we do know that in 1237 he travelled to York as part of the Scottish delegation who negotiated the Treaty of York. It seems he largely confined himself to his comital demesne, which would explain his conspicuous absence from the records, despite a relatively long reign. By an unknown wife/wives, he had at least five children: