Bob,
You have to take into account the time frame of the Mk4 retirement from the flying training role. In 1969 the first of 110 Mk5 aircraft was delivered as direct replacements for the T4 in the advanced portion of the basic flying training syllabus. Also at this time the basic flying training schools were going through an enormous closure programme which began in December 1966 when 7 FTS at Church Fenton closed, to be quickly followed in 1968/9 by the closure of 2 FTS at Syerston and 6 FTS at Acklington, that's half of the basic flying training schools closed in 3 years.
Linton, Leeming and Cranwell were the only ones left and there were sufficient T3 and T5 frames to equip them so there was no need to even consider a refurbishment programme for the Mk4.
The T4 lived on at CATCS Shawbury flown by Marshalls crews, the School of Refresher Flying at Manby and the JP Sqn of 6 FTS at Finningley as well as the small number at Chivenor. A total of less than 50 aircraft which could be found from the overall Mk 4 fleet.