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Hey John, I have some bad news..

Given your house main is tripping first.

It sounds like your house main is only 60 Amps, so you don't have a lot of room for any extra pull, and even giving 33% to the water heater might be too much, remember that your total available power is about 60*240 or roughly 14000 watts. Giving 4500 to the heater is a lot! It is about 33% of total available. Add a dryer 4000, dishwasher 1500, stove 8000, and suddenly POP! you're over without even having the tv on...

Another idea, you said you had 6 wire, this sounds like the wire from the main to the box inside, but not the wire from the inside box to the heater. A typical 3700 heater can be installed with 12 wire, so you might have 12 wire in place for the old heater and now you'd be under. You MUST be using #10-3 w/ground wire per NEC for the new heater! If you are using 12 wire, you can't step up to the bigger heater without changing the wire! The wire might actually be heating up and slowly increasing the load on the line, although I doubt this, if the wire is a 14 wire though, it could easily load up...Check it out..

The heater should operate under a 20 Amp breaker without failure. If you tried a 20A breaker and the house still kicks out first, you know it isn't the heater.

It sincerely sounds to me like a potential problem with line ratings/overloading. After the thing kicks out, see if the wires are warm or even hot to the touch, this would be indicative of a problem with using an underrated line.

The guy that said you added 3 amps of load is correct, but he didn't mention that the 3 amp addition is eating another 5% of total available load on an already burdened service.

If you are marginal on available load, consider changing all incandescent lighting in the house to flourescent bulbs, they last longer, and draw a lot less load. It sounds like you don't have natural gas available either, too bad, it would be a great alternative for a 60 amp main, and its cheaper.

Finally, sorry I rambled on this answer, you can downgrade the element from 4500 to 3700. It'll take longer to heat the water, and it may void your warranty, but you might not have a choice, all things considered.

It is pretty late and I've been known to make a few late nite mistakes, so hopefully someone else can render an opinion my posting.