Customs is fine, unlike the US you'll just walk through, unless you catch their eye. No forms to fill out, no need to speak to them normally, no queues.

As long as you don't need a visa for the UK, immigration probably shouldn't be too bad around that time. I went through T3 security at around 9am last week, there was no queue for the e-passport gates (EU citizens with chip passports only), and the other queues looked pretty short too. It was a hell of a lot better than the last time I was in T5 around then!

Land-side, from T3 to T4, there's a free train every 15 minutes. Alternately, if you have an oyster card / contactless, it's free if you touch in on the Tube in T2/T3, ride the train one stop towards London, then ride another back to T4 and touch out. You'd have to pay if you did it on a ticket though. From T3, with the need to change on the tube (it's a one-way loop from T4 to T23), I'd suggest following the signs to the Heathrow Express then take the free shuttle train to T4.

So, with a long layover, I don't think customs+immigration+transfer should be a big reason for stopping you visiting the arrivals lounge, if you want to try it!