Tywyn 4 is currently located inside St Cadfan's Church (NPRN 43861), at its west end near Towyn 2 (NPRN 302689). It was first noted in 1986 after the demolition of Ynysmaengwyn mansion (NPRN 54223), where it had been built into one of the outhouses. It was originally a sundial and is thought to have been associated with St Cadfan's Church. Its primary inscription is thought to be eighth to ninth century in date. In the eighteenth century it was reused as a milestoneand and almost certainly stood on the A493 where it passes Ynysmaengwyn. A milestone annotated with 'Towyn...1' is depicted on historic (1888 and 1901) Ordnance Survey mapping on the road near Ynysmaengwyn, some 760m north-east of Palmal. In 2010 it was erected outside the leisure centre on Tywyn High Street.

The stone is an unshaped, roughly quadrangular pillar, carved on one face only. In the centre top of the pillar is a gnomon hole, and the remains of the vertical dial (the lower half of the circle) emanate from this. Immediately below are two shallow, incised circles. The secondary inscription dates from the pillar's time as a milestone and consists of the words 'From Tywyn 1 Mile'.

A further incised stone,Tywyn 3 (NPRN 419548), is thought to be seventh to ninth century in date and is built into the tower. Tywyn 1, now lost, dated to the fifth century and is also associated with the church.

Sources include:
Edwards, N. 2013, A Corpus of Early Medieval Inscribed Stones and Stone Sculpture in Wales: Volume III North Wales
Ordnance Survey, 1888, first edition 25in
Ordnance survey, 1901, second edition 25in

N Vousden, RCAHMW, 11 November 2013