I have 4 PX4’s on my 5 at the moment, but 2 will need replacing soon.
Is it okay to put PX4s on the front and put the new T1R’s at the rear?
I beleive the T1 is supposedly a higher perfomrance tyre than the PX4.
Thanks in advance 
My local garage (whom I trust implicitly) told me that matching tyres in sets (ie two front and two back) is ok, just don’t mix brands on the front or back either side as this can give different behaviour to each wheel under braking.
I have Toyo TR1s and can recommend them to you, try Camskill if you have 15" wheels, only £23 a tyre!
Definitely avoid putting tyres which are different in any respect on opposite sides of the same axle.
What your garage says about slightly different tyres front-to rear is true for most cars - it is perfectly legal and usually works OK. However, unless they are very familiar with the MX-5, they may not be aware that these cars are unusual in that, to get them to handle at their very best, they usually benefit from identical tyres all round. There have been plenty of posts in these and other MX-5 forums where owners have complained of slightly odd handling, and where the cause was found to be non-matching tyres front-to-rear.
For these reason, it can be a good idea to follow the suggestion in the manual - to swap your tyres front-to-rear every 6,000 miles or so. Otherwise, you either end up with non-matching tyres front and rear, or regularly having to discard two tyres which are only partly worn, or are permanently stuck with always having to buy tyres of exactly the same make and type.
A PX4 of mine blew out, after only 2 months. Local Toyo dealers were unable to source a replacement, so I’ve ended up with an odd T1R instead
That ones a bit of a mess!
Is the T1R a replacement for the PX4 in Toyos lineup? Ive noticed that the PX4 isnt as readily available as it was.
Gonna check my tread depths properly and see what I need to do.
PX4 is the newer design.