My euphorbia wulfenii is looking straggly

[Linens]

I put one in 4 years ago and it’s always looked amazing and I’ve never touched it other than to take out the occasional stem but it’s looking scraggy this year and perhaps I should have been pruning it all this time. I think it’s too late this year but in autumn should I hard prune or is it all too late? Thanks for any advice

They are not very long lived so 4 years is pretty good going. They seed around quite a bit ( at least they do in the garden I'm currently in!) Here (SE England ) they are just coming into flower. Any dead stems you could cut out now, and then cut the faded flowered stems out each year. They should produce new growth at the base to flower next year.