I'm building a "heavy" class freshwater baitcasting rod (12-25 lb mono rating, but I'll be using 40 lb. superline). Will #4 micro guides be appropriate for this rod? Will they hold up if "the big one" strikes?

Yes

Yes for sure.

I have 3's on the same rated rod for Striped Bass and it's held up very well.

DR

I catch stripers consistently due to the way I bass fish - all micros all the time.

Client of mine boated a 35# striper he caught on a river ledge dragging a jig. MIcros did wonderfully.

Another client is using micros on a tarpon rod. 100#+ fish. No prob.

Chuck Mills just posted about a musky on a micro crank rod - No prob.

MIcros just placed 3rd in the IFA out of Grand Pass, LA last weekend - 7 - 8# redfish - no problems.

So basically.... Yes.

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AD

Thanks. It seems that all are in agreement, i.e., YES.

Just remember, If you are using braid and are going to tie on a leader, the guides will have to pass the knot.

Closing out on the third season of using them on flipping/pitching sticks with braids to 50lb and no failures.

Not saying implying you can't, but If a person can't get a leader knot through 4mm guides with the size lines you mentioned?
I would suggest finding someone to teach them how to tie knots

I'm still not using braid, yet! But, For 17# - 20# mono or fluoro I'm using 3mm and 4mm AmTak and 3.5mm Fuji's with not a single problem. If fact, I'm throwing an 8' crankbait rod this weekend with 17# fluoro with all 3.5mm Fuji's on top and not a single problem.